WordPress refers to the GPL as their Bill of Rights, but we like to think of it as a free software license. The GPL is a popular licence that grants the recipients of a computer program the rights of the free software definition, and uses copyleft to ensure the freedoms are preserved--even when the work is changed or added to.
Matt Mullenweg of WordPress writes, "We’ve always done our best to keep WordPress.org clean and only promote things that are completely compatible and legal with WordPress’s license." But just to be double sure, he recently consulted the Software Freedom Law Center for their opinion. Mullenweg's results were summed up on the WordPress blog in one simple line:
"PHP in WordPress themes must be GPL, artwork and CSS may be but are not required."
In order to recognize and celebrate a few of the folks creating 100% GPL themes as well as providing support and other services around them, WordPress has a new page listing GPL commercially supported themes here.