I think this is actually a bigger deal from a developers perspective than most of the other announcements. Since it's a compiler feature, does anyone know if it's compatible with older os releases?
Once you're familiar with the manual memory management conventions, it's not difficult, but this should make iOS development more approachable.
"ARC is supported in Xcode 4.2 for Mac OS X v10.6 and v10.7 (64-bit applications) and for iOS 4 and iOS 5. (Weak references are not supported in Mac OS X v10.6 and iOS 4)."
Any new platform that isn't ultradoomed is interesting. I might have been waiting for iOS development to become less brittle before getting involved (that is, if their policies towards developers and users weren't so repellent).
Once you're familiar with the manual memory management conventions, it's not difficult, but this should make iOS development more approachable.