At long last, the holy grail has been achieved. PlayFair is the tool to remove the DRM from your iTunes Music Store purchases.
I am a bit late to report this, because I had problems getting it to work.
It turns out that the code in playfair that attempts to locate your iPod is a bit buggy. You can help it out by setting an environment variable IPOD to the path to your ipod (yes, you currently must have an ipod, and it must be mounted and have copies of all the songs you wish to decrypt).
If you're using BASH as your shell, export IPOD=/Volumes/blah_blah
If you're using tcsh as your shell, setenv IPOD /Volumes/blah_blah
(where blah_blah is the name of your iPod. You may have to quote this path or escape spaces and special characters).
Then simply use playfair as directed: "playfair m4pfile m4afile".
Note that theoretically there is no need for playfair to require an iPod. Some sort of keying data is kept in "/Users/Shared/SC Info". Perhaps someone will fix playfair to use this source instead of the iPod.