Edition #8 of Podcast User Magazine has a great article by Skinny White Boy on the effective sourcing, creation, and use of music beds.
Beds, for the uninitiated, are clips of music suitable for background noise while a podcaster is speaking.
We use intros/outros and segment bumpers during The JaK Attack!, but nothing under the segments themselves. I did this waaaay back in the first 8 or so shows I made in early 2005, but my listener feedback was so incredibly against it that I stopped and haven't looked back. I likely got that response because I didn't know how to use beds properly.
Some of Skinny White Boy's tips:
Only use instrumental music. Don't compete with a singer
Watch the volume. You know what you're saying so it's easy to understand it. Your listeners don't know what you're about to say and will have a harder time with it
Only use what you need. Don't play 4 minutes of music when all you need is 10 seconds.
That's just a little bit to whet your appetite. The article is 2 pages long and contains more tips and sources to find appropriate music.