Missing Contributor License Agreement #3

Dave, you should probably add a CLA (Contributor License Agreement) to the project if you're considering re-licensing the code in your repository under different terms to different parties. If you haven't gotten your contributors to sign a CLA first then you should get their approval every time you re-license, which may not be possible.

There's another GitHub project that may make it easier to add CLA signatures to the project. I'm haven't tried it out yet, but thought I should pass it along:
http://jayunit.net/2013/01/09/clahub-easy-contributor-agreements-on-github/

Readium has a similar open source strategy, where the repositories are distributed under an open license but proprietary re-licensing is considered on a case-by-case basis. You can read their CLA here:
http://readium.org/licenses/icla