Mike Vernal, the PM on Indigo has a good article on what is a PM at Microsoft.  Some relevant points:

• He describes himself as a Technical PM, but that his role is essentially one of being a therapist.  He makes sure that people talk to each other.
• The PM role is quite MS specific.  He notes that it's interesting that MS spends so much money on PM's essentially to make sure that people are talking to each other.

" ... the common factor amongst all PMs -- PMs are facilitators of communication. The more gregarious, inclusive, and communicative a PM, the better. I thought I would have spent a lot more of my time on technical design work, but I'm actually ok with the communication schtick. At least it's not lonely."