The new iTunes Plus removes the copy protection from almost all of its songs, but all files downloaded still contain the email address of the iTunes account used to originally download them.

This would make it very easy to track down the original purchaser if the songs end up on a file-sharing network.

From cnet:

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.

You can find a lot more in cnet's article, iTunes Plus: Everything you need to know [crave.cnet.co.uk]