I can't, since I don't know any more than the next guy reading the docs. I'm sure there'll be an in-depth WWDC session on this and I'm eagerly awaiting the videos.
(I haven't done any Mac OS programming with GC, so I really don't know anything about it, but I've always heard people say the GC is not all that great. So my own uneducated guess is that while GC is messy and complicated, ARC is simple & doesn't have any runtime overhead (they say in the docs that retain/release is now much faster, as is autorelease using the new syntax).)
(I haven't done any Mac OS programming with GC, so I really don't know anything about it, but I've always heard people say the GC is not all that great. So my own uneducated guess is that while GC is messy and complicated, ARC is simple & doesn't have any runtime overhead (they say in the docs that retain/release is now much faster, as is autorelease using the new syntax).)