@Jon Brown -
I subscribe to the untested legal opinion that calling functions from GPL code (WP) makes a work derivative. I know plenty of people disagree with that, however I think that for a work not to be derivative (of WP) it needs to stand on it’s own. Stand on it’s own, as in it could run from the command line or along side Drupal or along side whatever. In other words the plug-in (or theme) would get an input, do it’s thing and then return an output without calling any functions from GPL code (WP).

That opinion is just simply not supported by extant copyright case law. There is no legal basis whatsoever for claiming that dependent equals derivative.

The legal standard for a derivative work is incorporation, in some concrete form, of copyrightable content of a protected work.

WordPress could sue Thesis for making WP function calls. Thesis could sue you for making Thesis function calls. In both cases, the use of function calls would be ruled non-copyrightable, and therefore non-infringing. And in both cases, even if we go with the legally unprecedented assumption that the use of function calls was copyrightable, in both cases the rules would unquestionably be ruled fair use, and therefore non-infringing.

So, I agree with your original statement regarding being ecstatic that Thesis has gone split-license GPL - but this turn of events doesn't really impact the underlying question regarding Themes being inherently derivative of WordPress.