after Dark Sun...so, maybe Spelljammer? hey we can but hope! :p
I think it will be something a bit more 'conventional' than Spelljammer. While I think Wizards can take more chances with settings now that there's only three books for each of them, I'm still inclined to believe they'll go with something that's closer to mainstream fantasy in the year after Dark Sun comes out. Partly that's because it leaves Spelljammer around for next time they want to publish something really unusual. Otherwise there aren't many truly unusual settings left to publish among the existing ones.
On the other hand in interviews they've stated that when they do expansions they prefer not to do ones that are too close to the core system. They want each expansion to actually add a lot of changes to the game world so that people aren't confused about why they need to exist. Spelljammer is definitely something different from the norm...
I love Spelljammer and would want nothing more than to see them rerelease the campaign. However, I think Planscape would come out before Spelljammer- don't ask me why. Just my hunch.
Yeah, you're probably right. 
One idea I keep hearing is that WotC could merge Spelljammer and Planescape.
One idea I keep hearing is that WotC could merge Spelljammer and Planescape.
That is only fan speculation as far as I know.
Mind you, I know that since I'm one of the fans speculating it, but if it has come from an even semi official source, I haven't heard it.
As for how likely it is... after spending the past few months researching what the settings actually were and not what they were to outsiders I can say it is possible but no longer the surefire thing I thought it was to begin with. Spelljammer was highsea/space adventures, traveling the space between primes and doing whatever. Planescape was all about belief having an effect on the universe as a whole, with it's various factions ignoring the primes and sticking to the planes. They don't line up the way I thought they did before, but they also don't contridict each other so it is still up in the air.
Ultimately I think they offer a lot to each other, and I'm personally going to work on thinking on how they can be merged even if Wizards of the Coast won't be doing it themselves.
I've also seen a lot of people claiming that they're now core... which honestly is rather... well I can't use that type of language on the forum so I'll just say it's rather false. Both settings have been raided, but they aren't core. Sigil was taken from Planescape, but it wasn't core to the setting as much as the factions were. The name spelljammer was taken to be used in the manual of planes, but actually looking at it's stats it is kind of obvious they aren't spelljamming vessels as the Spelljammer setting knew them.
Once we get the Planes Below and the Planes Above, we'll see what hope either setting have of becoming the next setting. 2011 would be the year for the Planes Between, afterall. I'm still personally hoping for a merge though.
I'm...not really sure what to think about a merge like that. I don't know very much about Planescape so my knowledge is definitely limited here. I just hope that if they do merge them that they are able to separate the aspects of the settings as easily as you say. I'm not sure if I would want my spacefarers to start using their beliefs to effect the universe.
On the other hand, doing a merger like that would probably make it easier and more profitable for Wizards to come out with a manual that at the very least had more details on how they would like Spelljamming to be in 4E, and that would be alright with me. Whatever works the best for them financially is probably the most likely outcome.
I asked Andy Collins at GenCon if he was going to do a 4e conversion for Spelljammer (such as the one he did in Dragon for 3e).
There was no 'official' answer, but the impression that I got was that if someone in the community wanted to create a conversion guide for 4th and submit it to Dungeon/Dragon then it would be a good idea and that it was not something he was planning. Again, that was reading (hopefully incorrectly) between the lines of what was said and watching body language.
As far as it being a released campaign... I came away disssappointed. Again, no official yes/no but the question did not seem to be received in a positive light.
I for one would be very interested in Spelljammer in 4e... MUCH more so that freaking Robots in my D&D 
...
Well with the Book the Plane Above (it is about the Astral plane) it will sort of be like Spelljammer has come back. With the Manual of the Planes, Planes Below and Planes Above it is like Planescape is back. Plane scape, aside from Sigil was never really a setting in the traditional sense. My money is on Ravenloft of a newby. Newbies aren't bad.
I disagree with that assessment. WotC might consider Spelljammer back but I don't. I don't care for turning spelljamming into planar travel. I don't expect WotC to do a Spelljammer setting.
Ravenloft has also been integrated in the core so I'm not sure they'll do that. My general thoughts are either Dragonlance or a new setting.
Maybe there is hope for Spelljammer. But it needs a new name. Like Star Sailors or something.
Perhaps they could call it Hackjammer!
I disagree with that assessment. WotC might consider Spelljammer back but I don't. I don't care for turning spelljamming into planar travel. I don't expect WotC to do a Spelljammer setting.
To be honest, I don't like the idea of PCs sailing around from campaign setting to campaign setting. Allows too much cheese mongery with people taking the best from each setting. It also creates the unnecessary need for a cross-setting metaplot.
I like the idea of characters visiting places like Sigil, Baator, and other island-worlds in the Astral Sea, while still remaining within the chosen campaign setting. It allows a lot more versatility than "Oh hey, you can sail from Eberron to Abeir/Toril now." The DM is more free to deny cross-setting travel while creating their own exotic wonders for everyone to partake of.
Also, as for players travelling from Eberron to Abeir/Toril... there is nothing stopping it from happening now. If the DM builds in a story line that allows travel from one to the other either directly, or through one of the outer planes, it will happen without any Spelljamming.
If the DM does not want that to happen in a campaign, then the players simply never find a portal from one to the other... the phlogiston is huge... nothing says the path between the 2 places is shorter than a 1000 year trip ;)
andnoted his use of it in his "Corsair" novel, which rocked
I don't see the problem with any cheese mongering... I have run Spelljammer in home campaigns before (and would like to again) and the players never tried to do anything of that sort. Myself and my players both enjoyed a cross-setting plot (was FR to Greyhawk at the time)
That sounds like a lot of fun. I think that can work as long as the voyage is seen as an epic thing, and not just a ferry between two settings.
And if the players do want to try cheese mongering, let them (accidentally) take the Hand of Vecna to Realmspace, open the door that allows the god to gain a toehold in the sphere and have The Company of the Challace arrest them thinking that they are Vecna Cultist. They could then demand that the PCs help put right the wrong they brought on the sphere (or face permanant banishment).
Also, as for players travelling from Eberron to Abeir/Toril... there is nothing stopping it from happening now. If the DM builds in a story line that allows travel from one to the other either directly, or through one of the outer planes, it will happen without any Spelljamming.
True. And the backstory of the Celestians creates an organisation that would do both.
If the DM does not want that to happen in a campaign, then the players simply never find a portal from one to the other... the phlogiston is huge... nothing says the path between the 2 places is shorter than a 1000 year trip ;)
I'd be more inclined to make PCs get lost (and go to the wrong sphere) than make something into a 1,000 year trip. You can also make the shortest route between two spheres a six sphere trek and discourage traffic that way.
As for Eberron, I would rule that (because it did not exist in an out of character sense during the time that the 2e product line was out) it is not part of The Known Spheres. So the players would not just "fly to Shardspace" because there would be no maps that could lead them there. Maybe they could discover a warforged NPC floating in the phlogiston and find out about Eberron that way.
Mind you MMV gives an "official way" to get between Eberron and other campaign worlds (and allows illithids to fly Nautiloid ships to and from Eberron). See my: Monster Manual V (3e) links Eberron to Spelljammer! thread over at The Piazza for more details.