The problem I see
When Microsoft hast to develop and support 5 totally different versions of Windows 8 it will have even more problems than Vista had.
Same IE on 5 Versions of Windows? Or do they do it like today. One for CE, one for WinPhone7 and two to three for Windows on Intel?
Even the track record for keeping .net in sync over different platforms isn't that promising.
Since my customers just started moving from XP to Win7, I'm not panicking, but I expect to run a lot of virtual desktops not too far in the future.