I received an answer from Creative Commons this afternoon:
Under the terms of the license, we drafted the termination clause (typically clause 7 of all of our licenses) to allow the creator (such as yourself) to release the work under a different license so, yes, you can apply a CC license to your work in certain forums and not in others.
That means I am free to release my music on CD using a Creative Commons license and on iTunes or other download services using the full copyright.
Explanation: both releases will be copyrighted, but the CD version will carry the CC license which is a rider to the copyright, giving the buyer additional rights.
From the Legal Code
Subject to the above terms and conditions, the license granted here is perpetual (for the duration of the applicable copyright in the Work). Notwithstanding the above, Licensor reserves the right to release the Work under different license terms or to stop distributing the Work at any time; provided, however that any such election will not serve to withdraw this License (or any other license that has been granted under the terms of this License), and this License will continue in full force and effect unless terminated as stated above.
I have talked about this to Jon as well, and he thinks he might release Transit 2 under the same license. This particular CC license allows sampling and mash-ups….
You are free:
• To sample, mash-up, or otherwise creatively transform this work for commercial or noncommercial purposes.
• To perform, display, and distribute copies of this whole work for noncommercial purposes (e.g., file-sharing or noncommercial webcasting).
Under the following conditions:
• You must give the original author credit.
• You may not use this work to advertise for or promote anything but the work you create from it.
• For any reuse or distribution, you must make clear to others the license terms of this work.