I recently started a new Eberron campaign with the 4e rules, after having played Eberron in 3.5 since its beginning.
My players sent me their background stories and one of them included the City of Brass in his background.
Now for me, Sigil and the City of Brass aren't in Eberron's cosmology, maybe because I never read about them or thought about planar cities in this world. But remembering the "Saying yes" philosophy, I don't want to outright discard it.
How would you say, could planar cities work in Eberron and its cosmology? Or is there any mention of them, I'm not aware of ? Specifically the City of Brass could be in Fernia, I think, or would a place on Eberron itself be more appropriate ?
Maybe Hellcow could also post his thoughts about this ? 
Thanks in advance!
I think that city of Brass was mentioned somewhere. Anyway, Fernia is a perfect choice. Sigil is harder. Eberron already has a planar metropolis,Syrania, the Azure Sky: " Syrania is a grand metropolis that serves as an important crossroads for planar trade...Epic denizens of every plane make their way to the Azure Sky to buy and sell in the universe's largest and most diverse marketplaces".Personally, i'd use whatever parts of Sigil I wish to use on Syrania or even use them wholesale with a few modifications.
How likely, would you say, is it to find a person/creature on Eberron, that has already been in the City of Brass.
If I remember correctly the people of Eberron have a high planar awareness (we know there are other planes, because we see their influence on the world), but little planar experience (travelling there). Or is that a 3.5 assumption, that's not true in 4e ?
By the way, does it mention anywhere in the books anymore, that high-level people are rare in Eberron ?