I havn't used much win7, but I got 1 running on my GF's laptop. To my understanding WMP12 streaming feature requires a windows live account or something like that which should only be compatibe with another WMP 12 or later edition whenever it will come out due to it's closed nature. While XBMC uses UPnP which is to my best knowlege is an open (source?) protocol.

Mind you, I had problem with WMP 12 to play UPnP streams from XP (XBMC). It's amusing that Win 7 can't acces my shared files on my XP box because I don't have password on it. XD

Edit: I spend much of the night searching the web, but didn't find much resources on the topic.
Windows Live account only required for sharing videos online in WMP. But as I recall on LAN between 2 WMPs you need to set up some sort of home network thing(sorry I don't know what it's called) which involves typing in a passkey that is ganarated by one of your machines. I think this is the reason why it wouldn't work. I belive WMP is both UPnP and DLNA compliant though.
WMP 12 can play XBMC streams out of the box, just displays them in a weird way in the library.