From the 3.5 FAQ:
"Is a warforged considered to be wearing armor for the
purpose of using special abilities, such as a monk’s fast
movement?
The composite plating of a typical warforged doesn’t count
as armor. Certain warforged feats, such as Adamantine Body
(EBERRON Campaign Setting, page 50) specifically state that the
character is considered to be wearing armor, and thus would
limit use of such abilities."
