I just signed the contracts on a comic book deal for six of my stories with IDW. They got the exclusive right to sell commercial comics based on my stories, but those stories are already under Creative Commons Attribution/ShareAlike/Noncommercial licenses that allow fans to make non-commercial comics (and films, etc) from them. My agent, Russell Galen, wrote a nifty little clause spelling this out (see below).
Lots of people have asked me whether doing a non-commercial CC release makes it impossible to sell commercial rights to a traditional publisher. Here's how it can work:
The exclusive rights granted to Licensee hereunder are subject to a pre-existing Creative Commons license which grants members of the public the irrevocable and nonexclusive right to create their own adaptations of the Licensed Property, including comic books. Such Creative Commons-licensed works may not be sold or distributed for profit. Licensee acknowledges that under the terms of this Creative Commons license, members of the public may create comic book version of the Licensed Property for non-commercial distribution. Licensor agrees not to license the rights which are granted to Licensee hereunder to any competitor of Licensee or to any commercial enterprise intending to create adaptations of the Works for commercial distribution.