Hell, I'm still waiting for one of the big chains to come out with a hamburger with a fried egg on it.
I love fried eggs on burgers. The yolk has to be runny so it bursts on the first bite. Don't think I could say the same thing about peanut butter, though. It just doesn't sound right, although I do think I did see PB on a burger on an episode of Diners, Drive-Ins, and Dives on the Food network.