Oz, oddly enough, had a number of sentient constructs...the Tin Man not being one of them.
Tik-Tok(*), the Scarecrow, Jack Pumpkinhead, Scraps the Patchwork Girl, the Sawhorse.... I'm probably missing some; I'm not really an Oz fan.
They ain't warforged, but the principal's the same.
Point? Damned if I know.
But do the Warforged fit in with the Dwarves, and Elves, and all the rest? I see no reason why not. D&D is a genre all its own, and it's been evolving all along. There's a dozen different ways to make the Warforged fit, and my only objection is the PHB will be tying them to a specific background, and won't be making them "Generic".
Personally, though, I'm holding out hope for the Changeling in the new PHB. I love those guys, and it's a pretty common archetype to boot.
(*)Originally typoed as "Tiki-Tok". Obviously, the Polyneitzchians have a construct.