If you selecting the 10.6 or 10.5 SDK (which I do have installed) in xcode 4.5.2 you get into all kinds of trouble with arclite.
I discussed this in here: http://www.rawmaterialsoftware.com/viewtopic.php?f=4&t=10759
Actually… interestingly the errors I get when I do refer to __NSConcreteGlobalBlock!
I switched to using 10.7 with 10.5 compatibility as Jules suggested, but… this means that the app is no longer running on 10.5. perhaps Jules is using these settings but hasn’t really tested this in 10.5? I’ll add a message in the other thread too.