I heard some comedian talk about this a while back. They have the SNL comedian (don't know his name) do Obama. He's good, and sounds a lot like Obama, has his expressions and manner of speaking down. Yet, so far, there really isn't something they can "jab" with constant jokes.
President Ford only needed to trip and that started a blizzard of jokes. Remember Chevy Chase?
Reagan they got with his age and caught his mannerisms perfectly and they were funny. I remember way way back them mocking JFK exaggerating his Boston accent. There was even a comedy album by Vaughn Meter making fun. Oh, and Bush is such a fuck-up, he is a comedans dream. It's impossible to compete with that
If, as time goes on, and we get to know Obama more, and he "trips" or does/says something that would be great material, you can bet the comedians will jump on it. Right now though, I think he's boring material for a comedian anyway.
TIS