Rumor from windows fanboy #1 (Paul Thurott) is that Microsoft is working with a third party to make it happen for now. Sounds similar to the old iPod on Windows solution where Apple licensed MusicMatch. I wonder if they are doing that to test the waters before having the internal Microsoft Mac team devote time to it.
As usual, Thurott is quite a bit off base.
http://www.microsoft.com/downloads/en/details.aspx?FamilyID=1fe7ea0f-3ad6-4137-8397-d412a3792c33That's beta 1 of the "Windows Phone 7 Connector" for OS X. Digging around in the package, it definitely came from the Microsoft Mac team, with signs of the Mac BU code all over the place. Since I don't have a Windows Phone, the program doesn't do much for me when it runs, but it does indicate it syncs from the iTunes and iPhoto libraries. I also found Zune device logos inside the program, so I may try and borrow a coworkers Zune to see what it does.
Definitely impressed Microsoft is putting in the effort to let OS X folks use their new phone. For media syncing, this puts the iPhone and WP7 devices in the first party supported category, while Android and Palm owners on OS X still need to look to third party solutions.