The information was posted on the Windows Vista Blog by Mike Nash, corporate vice president for Windows product management at Microsoft:

“And, as you probably know, since we began development of the next version of the Windows client operating system we have been referring to it by a codename, "Windows 7." But now is a good time to announce that we’ve decided to officially call the next version of Windows, "Windows 7."”

Why Windows 7? Well, Nash takes the matter from (almost) the very beginning, starting with the release of Windows 3.11 (version number), moving forward to date-related names such as Windows 95 and 98 and right to "aspirational" monikers like Windows XP or Windows Vista.

“since we do not ship new versions of Windows every year, using a date did not make sense.[…] Simply put, this is the seventh release of Windows, so therefore "Windows 7" just makes sense.”

We agree with Nash on the date issue. As for the "aspirational" names, we can all agree that this only worked for XP, while Vista was mostly found in “hasta la vista” contexts.