Daniel Jalkut wrote in his Red Sweater Blog that Microsoft will possibly use information from the iTunes-Library for their new iPod competitor.

The idea is that as a lure to switch to their service, Microsoft will offer to give you for free Windows Media versions of some number of songs from your existing iTunes library.

To get a list of the tracks you purchased, Jalkut built a little AppleScript which demonstrates the simplicity to get such informations:

tell application "iTunes"

every track of library playlist 1 whose kind is "Protected AAC audio file"

end tell