Brent Simmons’s notes on threading in Cocoa applications:

1. Stuff happening in threads is private and self-contained. Black boxes, train cars.
2. Communication between two things should be on the main thread, except in carefully controlled circumstances. Communication includes notifications and KVO, not just direct calls.
3. Using GCD or NSOperationQueue is way better than the old ways of doing threading.

via inessential.com: Some notes on threading.