This can be read on the Apple forums:
SOME NEWS:
guys, i mentioned before i use to intern at ATI in the apple group. I contacted some people there, they finally answers. They said it has to do with the memory id on the graphics chip. They are NOT the same on all cards, they use different vendors for the memory on the graphics card. The problem occurs at boot, that's why the freezing is random. Look at the MemVendorID for those that are freezing. They should all be the same.
They also told me they working on the fix and it will be released next week.
http://discussions.apple.com/thread.jspa?messageID=5805421#5805421
SOME NEWS:
guys, i mentioned before i use to intern at ATI in the apple group. I contacted some people there, they finally answers. They said it has to do with the memory id on the graphics chip. They are NOT the same on all cards, they use different vendors for the memory on the graphics card. The problem occurs at boot, that's why the freezing is random. Look at the MemVendorID for those that are freezing. They should all be the same.
They also told me they working on the fix and it will be released next week.
http://discussions.apple.com/thread.jspa?messageID=5805421#5805421