In a recent entry on Podcasts and Music, I discussed a tendency of some podcasters to use music in their shows without legal permission, and possibly without the possibility of compensating the owners of the music. I've since discovered that much of the podcast world is quite aware of this issue, and has found a way to add music to podcasts legally and properly. Many podcasters offer to showcase new music from bands, and many bands, seeking publicity, have granted rights to play their music. There's a lovely term for music that is (legally) safe to use in podcasts: “podsafe music.”
There's actually a distinct podsafe music industry right now, because the heavy hand of the RIAA has created a special niche for those who want more airtime and those who want music to mix with their speech. This thriving market is an exception to the general assumption that the RIAA's autocratic approach to copyright will suppress creativity rather than expand it. You can find this copyable music at http://www.podsafeaudio.com. Here's more general advice about finding podsafe music. And by the way, here's a sweet mashup of podsafe music and copyable Flickr pictures. I hope you won't be shocked to learn that the quality of the best podsafe music is quite similar to the quality of the best commercial music.
I would like to recommend another podcast to you called digital flotsam. Its creator, P. W. Fenton, in the past, used commercial music on his podcasts, and he's quite certain that in the process, he has lead people to buy some of the music he quoted. But he's currently doing the "legal thing." He took all his old podcasts down and will put them back up as he remixes them with all podsafe music. He explains all this in a wonderful rant, in his 27th show, which I think you definitely should NOT listen to first; enjoy any of his other shows and then try the rant, which is right on-topic here. Fenton's reminiscenences are very enjoyable, and he also uses fine judgment in choosing music and other audio to play for us. I really enjoyed his unlikely story about being a policeman on Christmas day.
There's also a much bigger niche for the podsafe musicians to attack, once their music is somewhat sorted and there are some good tools to access it. In many countries, people who make parties, bars, nightclubs (etc.) pay considerable fees in order to pipe music into their places. They would not have to pay such fees (in some countries), if they used only podsafe music. Podsafe musicians would get, at minimum, valuable exposure when their music is played in these places.