I actually enjoy having separate files for interface and implementation. It's a pain in C++ where you have to put your implementation details (instance variables) in the header, but in ObjC, headers are pretty clean.

I've noticed that my Java-based coworkers often create well-documented interfaces with only a single implementation. They claim to do it for testability, but Mockito can deal with concrete classes as well - I suspect that they secretly enjoy looking at 10 lines of code that describe what a class does :)