Well your work is copyrighted the second you do it. Its just that without the registering no lawyer will really take the case. It is still copyrighted, its just that with the registration that they are allowed to sue for in the $100 or thousands for each offence and as well get the lawyere fees paid. That the real reason to register, since it allows a more forcefull threat behind anything. Check out http://www.copyright.gov/ for the copyright laws.