But if you hear the joke and decide that it's funny and you want to record it, the joke is past. Can you rewind XM radio to go back and grab it?

Still, sounds like an interesting project. There are tons of times that I hear a song I don't know on college radio and I'd like to record a clip or record the announcer saying the title/artist/label. Sure beats trying to type it into my Treo as I drive. (should have bought voice memo recording software for it by now...)