I'm surprised that I haven't updated this section since playfair came out. No doubt everyone has noticed by now that the link in the previous entry is dead.
Playfair was renamed Hymn (Hear Your Music aNywhere), and then another fellow rewrote the whole thing in Java as JHymn. JHymn is now the tool to use, as it also supports a generic UI for removing atoms from the mp4 files it processes. You can download it from the Hymn Project website.
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.
Thanks to the MPEG4IP folks... Go fetch the MacOS X MPEG4 tools here. Using them you can delve into the secrets of the files iTunes makes.
Having done this, I've figured out that m4a and m4p files are really mp4 files. In fact, if you rename an m4a as an mp4, Windows QuickTime will play it perfectly, and as a bonus will extract a WAV file if you like.
Using mp4info, you can peer inside an MPEG4 file. On an m4a file, you see the excpected: a single AAC encoded audio track. On an m4p, however, you see an "unknown" encoding audio track.
These are command line tools, so you will need to play with the terminal to use 'em, but they seem darned useful.
In the meantime, if your goal is just to be able to share your m4a files (encoded from CDs or what not) with Windows machines, just rename them mp4 and play them from Windows Quicktime 6.1 (or, of course, extract the AIFF from them on the mac).
The new iTunes store is great, and I'd like to thank Apple for minimizing the DRM. Of course, you can strip the DRM by writing a track to an audio CD and re-ripping it. That much even Apple will tell you. But there are two more hacks beyond this:
Phase 1: Turning an m4p file into an AIFF. Quicktime (pro) won't do this for you, but it appears that maybe iMovie can. Open a new iMovie project, add an m4p file, then use the Finder and go into the Media folder of the project. You'll find an AIFF ready to use.
Phase 2: Turning an m4p file directly into an m4a without re-encoding it. That is, stripping the DRM completely out of the file. This remains the holy grail. It is likely that some actual software would have to be written to accomplish this feat. Whoever writes it can, of course, look forward to lots of DMCA based heat the minute they achieve this.