When it all comes down to it, I guess I agree with vice president of Windows product management Mike Nash, it "just makes sense." Windows 7 will be just that in production, "Windows 7," according to multiple sources, including Gregg Keizer over at Computerworld.
I am a bit surprised at the sudden decision to go with the build number as the final name of the OS, but there are some interesting customizations that could be had with such a name. One such customization could come in the form of turning the "v" in "Seven" into the number "7." As shown by this wallpaper, the "7" in Se7en sort of looks like a sideways "v," and could certainly make a pretty cool product package.
I would also imagine the change is coming simply due to Microsoft running out of ideas for arbitrary names (i.e., XP and Vista). Going to the build number makes more sense, but will it result in future name decimals such as Windows 3.1?