Ah; apparently:

http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/Multithreading/AboutThreads/AboutThreads.html

"""using the fork function [...] Applications that depend on the Core 
Foundation, Cocoa, or Core Data frameworks (either explicitly or 
implicitly) must make a subsequent call to an exec function or those 
frameworks may behave improperly."""

Bah.