Windows Vista is due to be generally available in January 2007. But as most beta testers can tell you, it's going to be tough to make that date. Worse, for Microsoft, Apple is poised to release Leopard (the next version of MacOS X) at the same time. MacOS X is already a stable platform -- now running on Intel.
The challenge Microsoft faces is wanting to make its deadline (and not anger its Software Assurance subscribers with an additional delay) versus being sure that its next-generation version of Windows is truly ready to go.
Windows XP, which was largely a minor update to Windows 2000, was basically ready to go by May 2001. Windows Vista beta 2 and even the current build are not quite ready to be used full-time by most power users. It's getting there. Each build makes progress. But time is running out.
Mary Jo Foley at Microsoft-Watch has an article that goes into more detail. Neowin will have some more stories on this issue shortly.
News source: Microsoft-Watch