Themes are GPL.
Finally, a definitive source on the long talked about issue:
One sentence summary: PHP in WordPress themes must be GPL, artwork and CSS may be but are not required.
This matches with what I (and many others) believed so far. Also note that the post says that WordPress.org will only promote 100% GPL or compatible themes, which I assume means that you have to GPL your CSS and images too for your theme to be accepted there.