Owen Winkler, who leads the Habari project, makes an interesting case for how "...Habari absolutely slaughters WordPress" in the licensing department. He sums up his thoughts this way, which I think would raise just about anyone's eyebrows that works with WordPress.
"So that's it. Before you even get into the technical merits of the platforms, think about the license, especially if you're a theme developer. Slapping a Creative Commons license on your theme will work just fine for Habari. Doing it for WordPress is a violation of their license."
He says this because WordPress is released under the GNU General Public License that states that if you create a work that contains any part of WordPress it must "be licensed as a whole at no charge to all third parties under the terms of this License". Which is pretty much anything built "on top" of WordPress, right? Plugins, themes, small widgets, all have WordPress' built-in functions in them.
Should this be a real cause for concern? Owen thinks so. There is some interesting chatter in the comments of his post too.