ergophobe, this link you pointed out [wordpress.org...] is key. The guys at FSF know what they're talking about!
The PHP elements, taken together, are clearly derivative of WordPress code. The template is loaded via the include() function. Its contents are combined with the WordPress code in memory to be processed by PHP along with (and completely indistinguishable from) the rest of WordPress. The PHP code consists largely of calls to WordPress functions and sparse, minimal logic to control which WordPress functions are accessed and how many times they will be called. They are derivative of WordPress because every part of them is determined by the content of the WordPress functions they call. As works of authorship, they are designed only to be combined with WordPress into a larger work.
In conclusion, the WordPress themes supplied contain elements that are derivative of WordPress’s copyrighted code. These themes, being collections of distinct works (images, CSS files, PHP files), need not be GPL-licensed as a whole. Rather, the PHP files are subject to the requirements of the GPL while the images and CSS are not. Third-party developers of such themes may apply restrictive copyrights to these elements if they wish.
I'm sorry but if you based your whole business on extending and distributing GPL'd software and you didn't understand the GPL, that's your fault. You took a risk and the free software community will defend it's work from all enemies, foreign and domestic. A lot of times, when these things come up, people who have never heard of the GPL before get a really negative impression of the GPL - like it's some kind of virus that wants to destroy this nice programmer's ability to make money on their work. That's not at all the case - the onus was on the makers of Thesis to determine if what they were doing was compliant with the license of the software they wanted to extend.
That said I don't see why the guys at Thesis couldn't make plenty of money while embracing the GPL. They are not required to distribute the software for free. They are not required to make their CSS and images free. They can still charge for distribution, documentation, and support, but the code must be open and people must be allowed to extend it. Red Hat has made piles of cash this way for example.
[edited by: physics at 12:03 am (utc) on Jul 22, 2010]