Yes. You'll have to decrypt them first.
Mac only. I can't help you if you use Windows, I don't have a Windows box to test it on, nor the interest in using Windows. This falls under fair use which as far as I know is allowed on this forum. If not, mods please delete.
Prep:
1. Download Requiem 1.7.4 from The Pirate Bay
2. Download the key backup and restore scripts from
http://www.box.net/shared/ixq0bfrqcg
3. Backup your ~/Music/iTunes/ folder. Copy it to an external harddrive or to another location on your harddrive such as ~/Desktop/
4. Delete ~/Music/iTunes/ if you copied it, but if you just moved it to your desktop, you won't have to delete it.
5. Downgrade to iTunes 7.7.1 (this would be another whole tutorial, use Google.)
6. Add one of the audio books, songs, movies, or TV shows you've purchase and make sure it plays in iTunes 7.7.1. You may have to reauthorize your computer.
7. Use the Key Backup script that you downloaded earlier.
8. Move the iTunes library folder you backed up earlier back into its regular location at ~/Music/iTunes/
9. Upgrade to iTunes 8.0.1. Your library should load automatically. If it didn't, just reimport everything.
Okay, everything is prepped. time to decrypt. You will need to follow the next steps every time you want to decrypt something or a group of things. iTunes 8.0.1 automatically replaces the decryption key files, so you'll have to restore them with the restore script, otherwise you'll get errors when trying to use Requiem. Or theoretically you could just set the files as read-only, but I haven't tested that. Suppose I should. Oh well.
Decrypt:
1. Use the Key Restore script.
2. Locate the "mac" directory in the Requiem 1.7.4 torrent you downloaded. It comes with all versions. You'll see three things: decrypt, decryptall, and RequiemDroplet. We're going to use the RequiemDroplet.
3. Find the audio book you want to use on your Zune (oh, I just realized that you have a Zune and this entire tutorial will be useless to you because you probably use Windows. Oh well, I'll keep typing, I've already made it this far. EDIT: Just read your post again and see you have Leopard. That boggles my mind, how do you get stuff onto your Zune?) in iTunes. Right click on it and click "Show in Finder" ... this will take you right to the file.
4. Rename the file from *.m4b to *.m4p (Requiem doesn't support *m4b for whatever reason, so we're going to trick Requiem into thinking this is a protected song, which is *m4p)
5. Drag it to the RequiemDroplet. It will decrypt and place the decrypted file right next to the original file. it will be called *.m4a
6. Sync that *.m4a to your Zune and call it a day. The Zune supports AAC audio.