From an article on Cnet UK:

Be warned: your account information is stored in every file
Although iTunes Plus files feature no copy protection, files downloaded still contain the email address you have registered with iTunes. So although files can physically be shared with, and played by, friends and family, any of your purchases that end up on file-sharing networks, for example, can be traced back to you.

If you’re interested in an easy way to check your own files, find an iTunes Plus file on your computer. Then choose to open it with a text editor (Windows Notepad works fine). It’ll take a while to open and will appear to be full of nonsense text, but if you choose the ‘Find’ option and type in the email address you have registered with the iTunes Store, you’ll find that your DRM-free music is not personal information-free.

I realize it’s a small concession to make to the music industry’s piracy concerns. But we wouldn’t be as comfortable with an identifying code being embedded in a book or magazine. And remember, they are asking customers to pay more for a product that remains inferior to the free version.