There has been considerable debate (and more debate) recently about whether or not WordPress themes are or should be GPL-licensed. The official word from WordPress and the Software Freedom Law Center is that WordPress themes are only partially GPL-licensed. That is, the php code falls under a strict GPL license, while the CSS and images do not. Put simply, this is really f*&Ked up!
From my non-legal-background reading, this means that if you intend on selling a WordPress theme, you could be on shaky legal ground unless you package the CSS and image files separately from the core theme files. This would make the brand new Theme Installer utterly useless for out-of-the-box proprietary themes.
Personally, I believe that people should only be in the business of selling services related to WordPress themes or plugins, not the themes and plugins themselves. It is simply outside of the spirit of the GPL to sell add-ons to GPL-licensed products. I realize this leads to some questions about how to make money off of creative themes and plugins.
The main question that persists in my mind is: Should people be able to make money off of the GPL-licensed core WordPress code? That is, should we be able to sell themes and plugins? The verdict is still out on that one.