Here is something that will need to be cleared. I did not hear nor do i think that WinRT will be back compatible with windows 7. There is no way that an IT department or ISV will write an app to use WinRT if it will not run on windows 7.
Microsoft loves sending mix messages, Steve S. just want up there and pimped Win 7 then pimped win 8. Many companies just moved to win 7 from xp (not vista) and they normally skip a windows release. Anyway to make the point unless companies have the extra resource and a business need they will not be make apps just for Win 8.
This makes me think that for a while the best way to target all windows versions is .net 4 or Silverlight (which can run on xp, 7, and 8) and Mac in the case of silverlight;