Is NSUserDefault thread safe?
The Apple iOS 5.1 and OS X 10.7 documentation say that it is thread-safe; therefore it is thread-safe.
Speaking for 10.10 and iOS8 if you looking into the implementation you'll find that -[NSUserDefaults setObject:forKey:] is calling __CFPreferencesSetAppValueWithContainer, which will eventually end up in +[CFPrefsSource withSourceForIdentifier:user:byHost:container:perform:]. This method is using a pthread_mutex_t to lock the access to the dictionary containing the values.
So NSUserDefaults is thread safe.