Didn't we have this discussion back in Microsoft vs Adobe? I seem to recall that the conclusion was that look and feels were not protected by copyright.
Looks like he's stolen the name ("Yo Messenger") as well as the UI. I'm pretty sure he'll get his ass sued if Yo actually have any money (and if they care).
Ah no, it's just from an indie-dev :) But I was inspired by how Yo took communication to a whole nother level and thought about ways of how to push further what they started.