I'm having this issue as well after media center has extended to two of my 360's flawlessly for months. It doesn't want to stream music, videos, recorded tv, or even live tv. I called xbox tech support, he came to the long drawn out conclusion that it's my firewall on my router since we disabled my windows firewall during the call. Of course, my router hasn't changed it's setup for months (running DD-WRT perfectly for over a year). It will work if i'm wired in via ethernet, but not stream wirelessly.
UPDATE:
Installed a fresh copy of windows vista on a separate hard drive and attempted to stream wirelessly then, and it worked. So it's definitely not a router issue , not that I'm surprised.