Full details of the partnership between Nokia and Microsoft still haven't been disclosed fully, but Bloomberg has spoken to people "with knowledge," who claim Microsoft is paying Nokia more than $1 billion to promote and develop Windows Phone 7 handsets, for the 5+ years of the agreement. Microsoft is also paying for Nokia's patents, such as the Navteq maps.
It goes both ways though, with Nokia paying Microsoft for every copy of Windows it uses. Interestingly, these sources told Bloomberg that the $1bn cash-handshake was a tactic to make Nokia choose WP7 over Android. [Bloomberg]