The Microsoft purchase is an obvious one (i thought it would happen in 2011): pretty much the only thing Microsoft can do to keep Windows Phone a potential player in the mobile OS business.
Granted, Microsoft will now be competing head on with "partners" Samsung et all and might have difficulty getting other handset manufacturers to bundle Windows. However, the mobile device market is just too important and Microsoft must move aggresively to retain its industry leading position. The money involved is reasonable at $ 7,2 billion and considering the cash Microsoft has, its a steal.
This sure looks like Steve Ballmer's parting shot. An excellent one that makes Microsoft appear much less rooted in old style desktops and more focused on the future: mobile devices.