From Slashdot news:

The Doom 3 E3 Demo Alpha has leaked to the public. It looks promising, altough I'm only getting 12FPS on a Radeon 7500. Did anyone else have the chance to play it and could post their FPS? Here are some sites with more information and screenshots: here, here, here, and finally here.

John Carmack has already made a statement regarding the leaked demo, he's not happy about the situation obviously. One last thing, we would love to hear your comments on the leak, however no warez links please.