That Apple have married themselves to Thunderbolt (co-developed by Apple and Intel) may have had something to do with it. Previously Thunderbolt was not well supported on AMD platforms, as I understand it. This appears to be changing though.
Thunderbolt has been an open standard (and now royalty free) for some time. I'm sure if someone as big as Apple wanted to adopt AMD that was the only blocker, it wouldn't be a blocker for long.
