From Microsoft.com:

Today Microsoft Corp. announced the official name of its next-generation Windows® client operating system, formerly code-named “Longhorn.” Video of the name announcement can be seen via the link below.

http://www.microsoft.com/winme/0507/25234/Win_Name_MBR.asx

More info on Vista can be found at:
http://www.microsoft.com/windowsvista/default.mspx