Hey all, I checked out the copyright laws about all this.
And I found something promising but, I am waiting on somone with a degree in law to back me up. It turns out that, theoreticaly, any completed work is copyrighted already, provided that you can give proof of authorship. The copyright office is there so that you can have an official place of registry. That translates into less fees out of your pocket if somone infringes on your copyright. And, I don't know how to post links even if I could, but, if you google U.S. copyright office, it will give you any info on how to get one for your work. For a literary work such as a screenplay, it is only 33 dollars.
Also, after reading through the boring stuff, I think that posting on CF actualy helps you prove your authenticity. Your login name can fall under the 'pen name' rules, and every post is dated, that's pretty much all the office needs to get things going, once they can trace it back to you. But, like I said, I am waiting on a lawer to verify my theory.