In a post at the official Windows Vista Team Blog, Microsoft VP Mike Nash announced that Windows 7 will in fact be the final name:
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."
While I know there have been a few cases at Microsoft when the codename of a product was used for the final release, I am pretty sure that this is a first for Windows. You might wonder about the decision.
The decision to use the name Windows 7 is about simplicity. Over the years, we have taken different approaches to naming Windows. We've used version numbers like Windows 3.11, or dates like Windows 98, or "aspirational" monikers like Windows XP or Windows Vista. And since we do not ship new versions of Windows every year, using a date did not make sense. Likewise, coming up with an all-new "aspirational" name does not do justice to what we are trying to achieve, which is to stay firmly rooted in our aspirations for Windows Vista, while evolving and refining the substantial investments in platform technology in Windows Vista into the next generation of Windows.