Rapaire:
Exactly. You can assert copyright over something you wrote at any time -- the burden of proof is on whoever challenges your authorship. Copyright assertion gives you some protection, but full copyright protection has to be obtained from the Library of Congress's Copyright Office.
I'm not sure that that is quite true. If someone else registers the copyright before the author has done so then I believe the burden of proof would be on the author as he is contesting the record.