The next client version of Windows was originally codenamed “Blackcomb,” though the company renamed it to “Windows Vienna” in early 2006 and to “Windows Seven” or “Windows 7” more recently.
Windows Vista was a major release, and Windows 7 will be also be a major update. Microsoft is currently on a development path where every other Windows version is a major release, so it’s possible we’ll see a minor OS update between Vista and Windows 7.
Microsoft currently plans to ship Windows 7 in 2010, about four years after Vista. (Windows Server updates are on a similar cycle.)