Development of Creative Commons licenses

[Brian Boyko <brboyko AT earthlink.net>]

Hello, I have a question for you.

I'm about to start a site about roleplaying games - the tabletop kind.  
These are basically ideas, books, copyrightable stuff.

One of the projects I'd like to do is take the "nuggets" of the forum, 
and post them in a monthly or quarterly "best of" PDF.

I cannot do that without either taking the copyright away from them, or 
by requiring Creative Commons work.

What I would like to do is require those who sign up to the forum to 
agree to an "opt-out" creative commons license that has the attribution 
and share-alike provisions. If a person does not want a specific post 
under the Creative Commons licence, I'd like them to have to say that 
in the post "This work is not governed by the Creative Commons 
License."  If a particular poster (for whatever reason) wants to retain 
the copyright to ALL his work (including offhand comments) he can 
simply put that statement into his signature file.

I'll muddle through the technical details, but how would I go about 
finding a licence like that?