Actually, it's in Alpha. There is no official release, but if you look hard you can probably find leaks here and there.
And being in Alpha, it's not nearly stable. Originally, it's promises seem nice...but as they kept pushing the deadline, and cutting features (WinFS, a meta filesystem, was one of the two most hyped about new features...but now after pushing the release date two years, they can't even have that ready. I personally find that's it's beginning to look less and less appealing. Apple's OS X Panther already has a handful of the features Longhorn is supposed to have. It's fairly likely that Apple will have released two new updates to OS X by the time Longhorn comes out. It's definitely going to be an interesting time, 2006.