So a dog fight has broken out in the Wordpress community, if you haven't noticed.
Matt Mullenweg, after review by various legal experts, is sticking to his guns that themes and plugins that "extend" Wordpress violates the GPL if they are not themselves distributed under the GPL. Matt has gone so far as to post this on Twitter:
Just to reiterate, if anyone wants to try a premium GPL #thesiswp alternative I'll buy them a copy. [ma.tt...] Please RT
Hey @photomatt, I'm no lawyer, and I never said anything about my understanding of the GPL. All I said is that it does not apply to me.
So Andrew Warner got them together for a face-to-face debate.
Matt's position: the license is clear and if you want to extend WP, you have to abide by that license, and premium themes are covered by that rule. Other premium theme authors like Brian Gardner have gone GPL and Thesis should too.
Chris's position: Thesis is not an extension or derivative of Wordpress and therefore doesn't need to abide by the GPL
I think just one way to test it is, you know, take a screenshot of a website running WordPress without Thesis and then take a screenshot of a website running Thesis without WordPress.