Xcode 4.4 is available (for Lion and Mountain Lion)
itunes.apple.com
itunes.apple.com
Mike Ash goes into some detail on his excellent blog: http://www.mikeash.com/pyblog/friday-qa-2012-06-22-objective...
Long story short, some of it is compile time, but some of it involves classes in Foundation (like NSArray and NSDictionary) implementing new methods (like objectAtIndexedSubscript).
iOS needs a base SDK update before those methods will be available there. Or you could monkey-patch them in with a category yourself in the meantime.
I also think it should be possible to support container (NSArray, NSDictionary) subscripting for foundation classes at compile-time for all versions even if the new functions aren't around. They just have to translate to the existing functions. I bet they do this too[1]
(upvoted you, it is a very informative link).
Edit: some more thoughts.
[1] No, they don't do that in v4.4. But maybe they will in a future version of Xcode.
I just tried and it works. Also search for "Objective-C Feature Availability Index" in the docs.
https://developer.apple.com/downloads/index.action
and now I have in my path clang 4.0!
Kudos to Apple for the self-contained command-line tools package download. Hopefully that'll continue with another update for 10.8.