In researching a bit on Wordpress and GPL, there seems to be one major controversy over the Thesis theme, with no legal jurisprudence to clarify the situation.
The Wordpress team’s position is that any theme that you can make for Wordpress is a derivative work of Wordpress itself, and therefore subject to the GPL’s restrictions and must be GPL licensed itself.
One of the major theme maker’s argument is that a theme wholly created by them (meaning no copied lines from Wordpress code) is their own work and merely interacts with Wordpress, and is not a derivative work of Wordpress itself.
Having read some of the back and forth, this really seems to be a gray area where reasonable people can disagree. That’s one of the reasons I personally avoid the GPL (I use Apache 2.0), but then again, I’m not saying that’s the right thing for anyone else, especially since I don’t run a business off my OSS.
I don’t envy you guys for having to had to make and justify the call for which license you want to use. Following in the footsteps of another project as a spirit guide might smack a bit of abdication, I hope that’s not the case.
Questions like this might be a good opportunity to make use contacts your board’s network brings you. There’s no substitute for firsthand entrepreneurial experience.