At the very least, it appears that your name is attached to each file.
This seems like a fine idea. People can do whatever the hell they want with their music, but if they share it widely, Apple and others know who to sue. (I'm not sure how the tech will actually shake out.. like all other methods, stripping this info out of the file for someone who's suitably determined would be a trivial act.)
This is the kind of tech DRM makers should have been trying their hand at all along.
This kind of water mark has been found since the beginning in apple's DRM'd AAC. It's also worth noting that the hymn folks who first started publishing anti-drm software geared at ITMS were leaving the user identifiable water mark info intact... Until iTunes updated and quit playing tracks that had been cracked and still had the watermark. : /
So in the end, their response to anti-drm made the anti-drm folks remove a tool in ITMS's arsenal.