There's no legal precedent, but if you want to read a ton of legal speculation on a related topic (some by informed people, some by those less informed), there's a pretty large body of writing on the hypothetical issues of whether proprietary Linux kernel modules violate the GPL.
That was a good read, thanks. It supports that WordPress themes are likely not derivative works, though -- dynamic linking, use only minimal non-complex interface code, are primarily unique code that could run on any platform (HTML/CSS/etc) with only a small amount of WordPress "compatibility code" as the paper calls it.
edit: here's a pretty thorough law-review article: http://web.bu.edu/law/central/jd/organizations/journals/bulr...