The true answer is "when it's ready". Just like ID games.
My best guess is that it's going to be out by next fall. That's probably just as reliable as it's going to get, even if you hear something from MS itself.
The way I figure it is that MS can't allow it to fall that far behind the proccessor curve, I mean they didn't take long to port stuff to itanium. But I suppose that's entirely different market. It will make MS look inept if it takes another year to port a OS to another semi-compatable computer platform. If it comes down to it they will end up sacrificing features or abandon full-64bit support and do the win95's half 16bit, half 32bit thing. Then fix the issues later with a service pack.
Would be a good contest though. Guess closest the real official release date (not when they ship) for Windows XP 64 and get a free 3000+ amd proccessor and motherboard. Then do a random drawing if a bunch of people get it right.