Agamon said:
There needs to be a, "Core: no, Eberron: yes" option. Unless that is a given. That's my opinion. Warforged are a big part of what makes Eberron unique. Don't let them bleed out of that setting.
Well...
While I agree that I don't want to see them in the PHB1, or for them to become a "standard" part of D&D to be sprinkled throughout all the settings willy-nilly, I do think there's room for them in
some settings outside Eberron. All you have to do is change the background and flavor.
For example:
In my longest-running homebrew, a world called Selion, there exists an elven empire called Tir Nallón. (Or "Tir N'Lon"; I still haven't settled on a spelling, and have gone back and forth.

) It's got many aspects of the Roman Empire, including a militaristic/expansionist mindset.
The most feared of the Imperial Elven military forces is the Bronze Legion. The Legion is made up entirely of volunteers who have sacrificed their humanity (elvenity?) on behalf of their nation. They are magically and mundanely bonded with, and into, a suit of armor. It becomes part of them, they a part of it: one entity of both organic and artificial origin. They are not dead, not alive, not undead. They have become, in effect, living weapons of bone and flesh, leather and bronze.
This concept predates Eberron and the warforged by a few years, but because the PCs never encountered the Bronze Legion directly, I never came up with a good mechanical way to represent them. It seems to me that the warforged would do the job just fine.