Well, this is a hopeful sign. Rush can give a great rant when he wants to, but he is usually careful not to start something unless he thinks he can finish it.
This might be another one-off joke, like his occasional joke in early 2009: What is the difference between God and Obama? (God doesn’t need a birth certificate.)
But it sounds like a bit more than that.
Rush referred to BO as an imposter in the White House on his show today.
http://www.youtube.com/watch?v=hv9q_nR9EBY&feature=player_embedded