This was ridiculously hard to track down. It's only mentioned in the high-level [iOS 5 Release Notes](http://developer.apple.com/library/ios/releasenotes/General/WhatsNewIniOS/Articles/iOS5.html#//apple_ref/doc/uid/TP30915195-SW54) (not OS X), and no specific Core Data document. Specifically:

> The performBlockAndWait: method supports API reentrancy.