Thought I'd pitch in...

I remember having a discussion on someone's blog a couple of years ago on the same topic (although centered around plugins more than themes). Matt left a comment in which he indicated his interpretation (everything that interacts with / uses Wordpress code should be GPL) and it was as a result of that that I changed the way my own product was structured to ensure it would be compatible with GPL.

I therefore have a "plugin" layer (licensed under GPL) acting as a bridge between my product and Wordpress. I could therefore license the code for my product (which functions as a standalone app as well) as I wished, without infringing upon GPL.

The situation from that point on has always seemed relatively straight-forward to me (I won't say anything definitive, because this whole area is a minefield), for both plugins and themes - license anything that interacts with Wordpress code directly as GPL; anything that doesn't, can be whatever you want.

For themes, that means images / css / custom javascript (as happened with thesis), and for plugins it means code with standalone functionality that does not interact directly with Wordpress code.

It does mean having to structure things in a certain way (for plugins, at least), but for any project where it's worth a separate license, it's probably best practice to do so anyway (eg. so you can bridge it with other software too).

PS. I do understand the viewpoint of those who say "it all should be GPL regardless", but you also need to consider that not all projects are Wordpress specific. Whilst such projects should be GPL-compatible (eg. via split licensing), they should not be GPL simply because Wordpress is.