A post by id Software's John Carmack to a thread called Doom 3 Alpha Leaked on Slashdot has confirmation of the fact that an early build of DOOM III has been leaked. Here's the post, which vaguely hints at the source of the release:

No, this was not leaked on purpose.

Yes, we are upset about it, and it will have some impact on how we deal with some companies in the future, but nothing drastic is going to change in terms of what support is going to be available.

Making any judgements from a snapshot intended for a non-interactive demo is ill advised.

John Carmack