I've heard of a couple programs to do this, but neither are free. The more popular one is called Sound Taxi and if I remember right costs something like $15. I haven't played with it, but I do know there are JAWS scrips available for it.
The trial version only converts the first minute of a song, but should give you a feel for it. More info about it is at:
www.hartgen.org
Google is also your friend. Another option is another purchasable program that will add another virtual CD burner to your computer, and then using iTunes you can supposedly "burn" the protected tracks to a virtual CD as if you were burning an actual audio CD in iTunes. I was unable to get this to work,though.
I too am still looking for a solution as I want to be able to play all my music on my PAC Mate Omni once I get it back. I also have a growing number of iTunes Store purchased music.
HTH,
Brian