Okay, I'm studying law in Australia, so it is different - we don't have a copyright registration service. Thanks for giving me the link. Evidently, in the US you do have this system:
http://www.copyright.gov/circs/circ1.html#cr
Note that it is still not necessary, and copyright automatically exists:
http://www.copyright.gov/circs/circ1.html#hsc
In any case, if I create a work, and you copy and register it, that doesn't offer you protection because if I can prove I created the work, then you will have to give up the copyright. The copyright belongs to the author - registration just makes if formalized in the US system.
Copyright registration does not legitimize copyright infringement.