Our own Erica was one of the first to discover
that personal information is encoded in DRM-free iTunes Plus files. If you want to remove that "watermarking," Playlist Magazine has a solution
: TUAW favorite audio editor Fission
($32) from Rogue Amoeba "can strip out the identifying information in an iTunes Plus track without changing the file's audio." Basically, you just open the file then resave it as AAC (Original Format, Lossless) and the non-original format identifying information is not saved. Unfortunately, you have to do this for each file individually, but if you don't want your name and Apple ID in the file, this looks like it'll work (though I haven't tried it myself).