We learned a lot about using 5.1 for XP and how that helped developers with version checking for API compatibility. We also had the lesson reinforced when we applied the version number in the Windows Vista code as Windows 6.0-- that changing basic version numbers can cause application compatibility issues.
So we decided to ship the Windows 7 code as Windows 6.1 - which is what you will see in the actual version of the product in cmd.exe or computer properties.
http://windowsteamblog.com/blogs/windowsvista/archive/2008/10/14/why-7.aspx
...as a post-graduate he was told he didn't have much longer to live...
Let this be a lesson to all - graduating is hazardous to your health!
To be a kind of moral Unix, he touched the hem of Nature's shift. -- Shelley