What's your opinion on that fair use argument? That's the only way I could see Chris having a legal leg to stand on. It seems pretty clear that Thesis is a derivative work, especially given that it extends a Wordpress class[1], but if a theme's use of Wordpress does fall under fair use, he wouldn't be violating the license.
It seems that if you buy the FSF's position[2] that linking from a proprietary program violates the license (meaning that linking is not fair use), Wordpress themes are definitely not fair use. But I'm not sure if that's been established in court.
[1]: http://apeatling.wordpress.com/2010/07/14/thesis-and-the-gpl...
[2]: http://en.wikipedia.org/wiki/GPL#Point_of_view:_any_linking_...