ArsTechnica: Microsoft to update Windows XP before Longhorn release: "...with this comes new concerns that Longhorn's own release date is slipping far, far away, perhaps as late as 2007 or maybe even 2008 (a guy in a bar last night said 2010)."

Considering that Longhorn is essentially the realization of the vision of Microsoft's next-generation "Cairo" operating system (tagline: "information at your fingertips") that was supposed to be released in the mid-90s, it is now clear that Longhorn is the world's biggest piece of vaporware (baring Xanadu!).

Salon's Scott Rosenburg talks more about this in an article from December that I found while looking for "Cairo" trivia.