Matt anounced today the results of a interesting research conducted by the Software Freedom Law Center at his request. Matt summarized everything by saying that “PHP in WordPress themes must be GPL, artwork and CSS may be but are not required”.
In the opinion of the Software Freedom Law Center, the images and CSS are generally not taken into account by WordPress core, who is licensed under GPL license, so they could be licensed under a different license. They concluded that
In conclusion, the WordPress themes supplied contain elements that are derivative of WordPress’s copyrighted code. These themes, being collections of distinct works (images, CSS files, PHP files), need not be GPL-licensed as a whole. Rather, the PHP files are subject to the requirements of the GPL while the images and CSS are not. Third-party developers of such themes may apply restrictive copyrights to these elements if they wish.
So in a theme we would have images and CSS with a proprietary license (of course, they can also be licensed as GPL) and the strictly GPL code, the theme, built on top of the WordPress framework. They specify before that HTML snippets were dependant on where it was included and thus was derivative of PHP code.
The WordPress team created a page to promote themes that have commercial support by adding a new branch to the Themes section of WordPress.org.