Some may argue this is not within the “spirit” of the GPL. I’m simply trying to discover what is “legal” within the GPL.

Yeah, me too. I'm not even interested in releasing non-GPL code, I'd just like to know the exact answer to it (if there is one). I originally took 'Beagle's advice as gospel, but with so many people having a different view on it I'm just not sure anymore. If it was just random themers jumping up and down I'd have not believed them. But when Matt M says something which contradicts this advice I received I tend to listen as I'm guessing he also received expert advice on the topic.

While interesting I don’t think this really answers any questions; at least not to any depth of satisfaction.

Yep, the conversation left me with more questions than answers too.

The GPL is explicit about what constitutes a derivative work, and themes/plugins are derivative work. The GPL does not prevent you from creating and selling themes that are not GPL licensed, but using them in conjunction with Wordpress (or any other GPL’d software) would be a violation of the theme’s non-GPL license.

Hmmm, interesting point. I'll ask him to clarify the next time I talk to him.

If someone wants to leverage a ginormous existing market that uses GPL software, playing nicely with the GPL is necessary.

That's the approach I'm taking too. And anyhows, it's nice to be part of the WP community :)

But, as the article said, the WordPress project (being a private org) is free to take any action that may discourage the use of non-GPL themes, such as pulling them from the directory, publicly denouncing them, etc. So it’s a risk vs. reward scenario.

Yes, that is a very good point to emphasise. I saw a lot unpleasant blog comments a while ago about WordPress.org breaking the law by not promoting non-GPL stuff, which is completely ridiculous.