Bob Stein has just posted that the next version Windows, currently codenamed 'Longhorn', will in fact be named Windows Vista.
There is currently no confirmation or announcement from Microsoft on this, but as I am in Seattle meeting some people from Microsoft at the moment I will be asking around...
Update: Microsoft has just posted their 6AM PST Press Release confirming that Windows: Codename Longhorn will be called Windows Vista. Microsoft has also launched their official "Windows Vista" web page. Announced on the page: "Beta 1, targeted at developers and IT professionals, will be available by August 3rd 2005".
Update: Steven Bink along with numerous other Windows enthusiast site admins have suggested we nickname Vista "WinVi". "Vi is the first two letters of Vista, and Windows Vista is Windows NT 6.0. In Roman Numerals VI is the number six." ActiveWin is reporting that the official tagline for WinVi is "Clear, Confident, Connected: Bringing clarity to your world."