My guess is this SDK won't be backwards compatible with existing phones. I expect this will be released at the same time as the next gen iPhone.
Not a chance. It would shoot their entire 'software based' device positioning in the foot. The only reason to orphan an existing platform would be that it doesn't have the processing power or memory to run a newer OS or apps, and I've never seen any manufacturer orphan the immediately preceeding version.
Check out the compatability list for Leopard that just came out. Compatability goes back quite a ways.
Edit: Correction... MS shoots their customers/partners in the back, not the foot. See the way Zune DRM is incompatible with existing licensees of MS DRM.