Well after struggling finding a name for the vista successor Microsoft decided to simply call it “Windows 7”. The codename becomes the actual name.
After all it’s a pretty reasonable choice, because for the enduser it is way easier to distinguish between the different Windows versions if the name is a plain number. Will Windows 7.0 be better than Windows 6.0 also knows as Vista ? 7 is a magical number, but actually it’s Windows 6.1 since Windows 7 will be more like Vista SE. Windows 7 will be completely backwards-compatible to Vista, there aren’t enough significant changes to justify a new kernel version 7.0.