According to a post on the WordPress Development blog today, there are finally some concrete GPL related answers regarding some fuzziness in the communities (and WordPress’s) understanding of exactly how themes fit into the GPL framework.
Long story short, the php (or at least the parts related to hooking into WordPress) of themes ARE required to be GPL compliant. The tricky bit is, the images and css are NOT. Meaning a theme could be partially GPL and still retain copyright of the artwork/design to some degree.
Here’s a snippet for you…
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.
If you want more details, head over to the post on the Dev blog for the full situation, including some more fun legalese type stuff.