Okay.
The most powerful group of craftsmen and smiths in Khorvaire is House Cannith, a semi-noble family who bear the Mark of Making, which (in addition to giving them a few nifty spell-like powers) can be put to use making wonders beyond the reach of normal men.
During the Last War, the living constructs known as Warforged were developed (some say, from ancient creation forges recovered from Xen'Drik) and sold to the warring nations as soldiers.
"Fought for a nation during the war" is likely to be the most common background for a warforged PC.
The war was ended with the signing of The Treaty of Thronehold. One part of this document concerned the warforged specifically.
In short:
-Warforged are classified as sentient beings, and therefore cannot be kept as slaves or "owned" warriors. (As slavery has long been illegal thoughout the five nations.) This was probably most strongly advocated by King Boranel of Breland, who had a loyal and trusty warforged companion named Bulwark during the war.
-House Cannith was required to dismantle their creation forges and stop building new warforged.
This requirement is officially part of the "warforged cannot be owned" thing, but it's almost certain it was included to limit the power of House Cannith. (Like modern day multinational corperations, the Dragonmarked houses wield extensive economic and political power even if they do not directly rule any nation.)
Anyway. This basically means no-one is supposed to build new warforged, so PCs from such a background will be no more than two years old.
However.
The nation of Cyre was engulfed by the cataclysm known as The Mourning four years ago. Cyre was home to House Cannith's leadership and some of their most important holdings, including a number of creation forges.
Now, it was believed impossible for someone without Cannith's Mark of Making to operate these forges, but there are rumours that the charismatic Warforged supremist known as The Lord of Blades has managed to use a creation forge to create new followers for his army, despite being a Warforged himself.
This is more likely to be an origin for new warforged villains, but since PCs are exceptional people, they could come from this background.
Also, as the forges were recovered from Xen'drik, it's thought there may be some warforged from the age of giants waiting amoungst the ruins. Lying inert for thousands of years, waiting to be recovered.
(Though the warforged of ages past mostly seem to have been radically different from the modern versions.)
This is also unusal, but could make for a good PC background.
Finally:
Show
A prominent member of House Cannith operates a creation forge in secret, where he experiments with creating new warforged. Sometimes he releases these new warforged into the world and watches what happens to them. This is also suggested as a source of new PCs.
