The two companies (Rovio and Mircosoft) has seem to had their differences in the past (November of last year). There was an occasion when the logo for Angry Birds was displayed on the Microsoft website without the consent of Rovio. Rovios reaction to that was to send out a Tweet stating that it was in no way committed to do a version of Angry Birds for the Microsoft Windows 7 Phone. Needless to say that the news was a disappointment to many fans.
In a recent interview, Rovio developer Peter Vesterbacka went on to state that there's no hard feelings between Microsoft and Rovio and that currently there are plans already in progress to bring Angry Birds to the Windows 7 Phone. Unfortunately a release date was not specified by Vesterbacka. When asked, he stated "Let's just say, Microsoft has a lot of lawyers" hinting that the hold up was mostly on the Microsoft side.
As a side note, Vesterbacka also mentioned that a 3D version of the game is in the works but did not specify on what platform or the potential release day.