I realize there is more to the deal than just the Nook Color/tablets, but I think it is interesting to imagine that either the Nook will continue to run Android and Microsoft will now be involved with selling Android devices or existing apps already purchased will not work on future versions of a non-Android Nook device.
Microsoft has a lot at stake - WP7, as it is now, lacks credibility and that's absolutely critical in this "post-PC" world. Nooks running WP7 (and abandoning Android) could generate some.