The folks on the Drupal board just released a new FAQ that answers many of the Drupal licensing questions that are out there. It's actually a pretty good primer into licensing for open source in general.
One interesting note is that if you use CIVICRM (which several of our sites do), the entire site falls under the Affero General Public License (AGPL) version 3 which requires that visitors to the website be given a copy of the server-side if they request it.
Another note, if a module or theme is created and distributed and it contains a Flash SWF file, it MUST include the FLA file when given to the customer. This is interesting because of the way some designers try to retain the source files of their work so that customers have to come back to them for different implementations of the same original graphic assets, e.g. "we want a different size of the ad you did for us." (BTW, we've always felt that if you paid us to create a some original code or artwork, you should have the assets. We retain intellectual property in terms of being able to re-apply the code to other projects but we give our clients full reign to do what they want with the code short of selling it. (That would fall under a different type of agreement).