That patent is on NeXTSTEP's NSDistributedNotificationCenter [1], which admittedly might actually be enforceable (I'm not sure of any prior art in this regard).
That said, after reading the claims of a lot of the other patents Apple is citing it is apparent that they are complete rubbish, for example [2] (the observer pattern applied to a desktop environment), [3] (generic proxy objects for IPC/RPC - Smalltalk/CORBA did it), and [4] (WTF???????).
1. http://developer.apple.com/mac/library/documentation/Cocoa/C...
2. http://www.google.com/patents/about?id=zIwLAAAAEBAJ