Guys hear me out...
I got the 2011 17" mbp and was having the same problem. Previously I had a 2009 13" and everything worked fine with my AppleTV 2. Once I got my new Mac up and running it worked fine then all of a sudden quit. It would show my Library, but I got an error loading anything (movies, music, photos).
I rebuilt my itunes library and it fixed the issue, but as soon as i rebooted/woke up from sleep it wouldnt work again. As crazy as this sounds all I have to do is connect my iphone to my Mac, reboot itunes, and everything streams fine! I dont know what the connection is, but it works. I have been doing this the past week hoping that a permanent fix will eventually come out. Give it a shot and let us know if it works for you.
Again, just hook up the phone so itunes sees it, shut down itunes and re open it, and it should work. Works EVERY time for me....