It seems that as part of the partnership, Nokia has struck a deal with Microsoft to allow for customization of the Windows Phone OS.
Pocket-Lint have the transcript of the briefing by CEO Stephen Elop. Here's the meaty part:
“We have the ability to do customisations and extensions to the software environment that are unique and therefore differentiate. It’s very important to understand this is not a standard OEM agreement. Microsoft is placing a big bet on us."
Earlier Elop had said Nokia had a "unique relationship" with Microsoft and that the deal was "not your mother's OEM deal with Microsoft." This hints at a very uneven playing field amongst the WP handset OEMs and it'll be interesting to see if if some of the other players express dissatisfaction.