Isn't it questionable whether WP7 will be manufacturer-neutral? The level of cooperation between Nokia and Microsoft remains to be seen, but a possible outcome is that Nokia ends up looking like a subsidiary of Microsoft.
But basically, I think RIM is screwing up so much that they'll be an acquisition target soon, and MS seems like the most likely buyer to me.
Does MS really need a brand with enterprise clout though? Doesn't the MS brand itself have enough enterprise clout?