But do you honestly think that Apple is encoding that specific information into each track sold through the iTunes store, making essentially every single track sold unique? Do you realize how much more expensive it would be to "imprint" that information into the audio?
Why, it's easy - instead of watermarking a DRM'ed m4p with the account information, which they are doing since the 1st day of iTMS, they add the same information to the unproteced m4a. I am pretty much sure it's just one call to a specific library function in the store code, called just before uploading the file to the user.
And for putting it onto the CD: I guess it might be possible to add coded informations to the CDA that are inaudible as essentially MP3/AAC works by cutting stuff from the audio stream that you won't hear anyway, so why not reverse this. It's only a few bytes of information, easy to hide and would be done by the AAC decoder in an Apple product (iTunes). Also, audio steganography is not a new concept at all.
But in this case, it would be easy to detect: Find a friend who buys the same track, burn both tracks to CD's, rip them back as uncompressed CDA files and run a binary diff on them - any difference is an indication for this theory.
Nevertheless, I still don't care, as I won't do filesharing.