Android may be open source but realistically how far can anyone diverge while maintaing compatibility, consistency, and the ability to push updates out in a timely fashion? At least with Microsoft they are entering a formal agreement. All these details have probably been worked out. Microsoft may have offered to let Nokia into the inner-circle of WM development. They may have agreed to have MS developers work very closely with Nokia for customizations. Hard to say without knowing all the details but it's doubtful Google would have agreed to the same terms since they simply don't need Nokia.

It reminds me of this story about K-9 developers wanting to re-integrate some of their enhancements back into ASOP and Google's response was basically 'nahhh' or Google giving Motorola sweetheart access to Honeycomb while leaving other tablet makers moaning & groaning on the CES floor. Those types of things probably contributed to Nokia's choice. Nokia wants to be actively involved in OS development. Their own efforts failed. Nokia being a somewhat old fashion sort of company was probably not comfortable with the uncertainty behind Google's rather blackbox-ish control of Android. In an ironic way Microsoft was actually offering more flexibility and openness of development plus all the perks of free marketing dollars, joint promotions, and generally being treated like a first class citizen with a contract to define the exact terms of the relationship.