It's not Longhorn anymore. Microsoft today announced that Windows Vista will be the official name of the next version of its flagship operating system, and that beta 1 of the OS will arrive by August 3.Source: MacCentral
Now we can stop the cattle jokes and start with an all new set. Yay. I can't wait.
They "received good feedback" by abandoning version numbers and release dates for their name for Windows. Hmmm, maybe its because Windows 95 was still in beta in 1995, same with Windows 98 and 2000. Of course XP does has a number, its NT 5.1.24. Just like Tiger is really 10.4 (the 5th revision of OS X throws people though, but that's another story).
And to quote sigma8 on the Macworld forums "They should have taken a cue from Nintendo.. and done something like Windows Blue, Windows Emerald.. and of course, Windows: Special Pikachu Edition."