From what I understand (and I am *not* a lawyer), in the USA you can sue
successfully for copyright infringement only if you've actually registered
the copyright with the Copyright Office. The copyright actually exists from
the moment of the work's creation, but it's not enforceable without the
registration. Thus, date-stamping schemes (online or
mail-an-envelope-to-yourself) won't hold up in court.

If I'm wrong, I'm sure I'll know in a few minutes.

Peace,
Paul