The reason: because when I open my WindowsPhone 7 solutions, then it shows them as "unavailable" - which makes me very very sad - because I have to use VS2010:
However, last night @dsplaisted asked me if I'd tried right-clicking on them and reloading them.
I can now open my Windows Phone 7 projects in VS2012 :)
Apparently, the problem was:
- I first tried to open this solution before the WP8 SDK was installed
- Visual Studio 2012 didn't know about how to open these - so it marked them 'do not open' in my solution options.