It WILL be out before the end of the year.
If it's not, Valve will not have the time between it and iD releasing Doom 3 that it needs.
Doom 3 should be out around March. I met Carmack at a convention last March, and talked to him about Doom 3 and its multiplayer (which turned out to be exactally what he told me it would be
) so I hope he comes to it again, so I can get a chance to see if they are putting any work into Quake 4, or if it's all just rumors.