[squarebear]

This is a popular myth. The poor man's copyright offers no legal protection at all as it's so easy to fake.
I could mail an empty envelope to myself, wait until a great CD came out then put the CD in the envelope and seal it and I would have what would appear to be a copy of the CD long before it was released.

If you are serious about your music, the only real way to do this is to spend money and do it properly. Also, keep copies of all your source files (the .flp or the reason file) associated with your tracks. That way you could show a court of law how you made the track whereas the person who was ripping you off would not have this.