There is finally an official answer to the question of whether or not WordPress themes must “inherit” the GPL license that WordPress itself uses. Matt asked the Software Freedom Law Center to examine the WordPress source and how themes fit in. The final, official answer to whether themes must be GPL? Yes and no.
The Theme Repository at WordPress.org will only host themes that are fully compliant with the GPL. But they do now now have a directory of other sites which provide “commercially supported GPL themes“. I expect that the combination of the license clarification and the commercial directory listings will stir a lot of new buzz in the WordPress theming communities.