You may have figured out your problem already, but here's my two cents...
I have had this exact problem in the last few weeks (sleeping media center won't wake up). There are two things I would recommend.
1) Roll back your updates to the last time sleep worked appropriately. You can do this via a system restore.
2) Turn off any software NOT related to media center. From the start menu, type msconfig, hit enter. Once you do this, go to Services, check Hide all microsoft services, then click disable all. Now, go to startup and click disable all. Save, reboot and see if that fixes your problem. Then, you can go back into msconfig and add things back one thing at a time and see who the offending party is.
Hope this helps!
Chris