Objective-C has only one, one global namespace. This is why you often see classes called SBJsonParser , so the class name does not collide with another JsonParser .
The general recommendation I saw is to prefix your classes with either your initials or multiple initials for the project you are working on, the name of the class, and then (sometimes) what the "type" of the class they are (like and the obvious convention for view controllers).
Honestly, I'm next to you, Mel, I would really worry about Objective-C to add some kind of namespacing function, at least something to sort the classes a little more (and a little easier).