I normally put Interfaces with the implementation however I can sort of see why you may want to keep them separate. Say, for instance, someone wanted to reimplement classes based upon your interfaces, they would need a jar/lib/etc with your implementation rather than just the interfaces. With them separate you could just say "Here's my implementation of that interface" and be done with it. Like I said, not what I do but I can sort of see why some might want to.
