Wow, what a stormy issue this has become! The following was going to be a follow-up comment over on Andy Beard's blog, but it got a little too long. Here's my take on the issue:

I guess I see it a little differently. Let me see if I can explain in a way that at least makes sense, even if it doesn't change anybody's mind :-)

First, the WP developers' code is, and should be, fully protected. If Chris were to take any files from WordPress and distribute them with Thesis without making Thesis GPL, he'd be violating their license. If he were to copy any of their code into Thesis, he'd likewise be violating their license.

Either of those would be essentially equivalent to static linking. Since PHP isn't a compiled language, there's technically no such thing as statically linked PHP. Essentially, if any code taken from WordPress were in the files being distributed with Thesis, that'd be equivalent to static linking. If on the other hand, the code in Thesis is unique, but just talks to whatever WordPress files are on the server, that's like dynamic linking.

But getting to my main point -- Chris' "moral" argument is that the WP developers get to control the license under which the code they wrote is distributed, and he gets to control the license under which the code he wrote is distributed. Unless the files he's distributing include code that they wrote, or code he created by modifying or "copying" code that they wrote, then to assert control over the licensing of the code he wrote is overstepping their copyright rights.

Since the GPL's power comes from copyright, it can't extend their rights beyond the control that copyright gives them. So I believe the LEGAL question comes down on Chris' side (assuming he hasn't in any way "copied" code from WordPress...I am not a lawyer).

The moral question is a bit more difficult to sort out. Clearly, the WP developers have no moral objection to people using WordPress on commercial websites, so it's not the fact that Chris is profiting that's the problem.

Reader Comment:
Is violating the GPL? – A fresh angle on the GPL vs. non-GPL wordpress themes and plugins | Hackadelic said:
[...] WordPress vs. Thesis: Can WP Themes Be Non-GPL? argues well about static and dynamic linking, and beyond. [...]
(join the conversation below)

Nor is the issue that, in broad terms, WordPress and Thesis are cooperating to deliver the blog content (if that were the problem, they'd also object to WordPress running on Windows-based servers, for example, since Windows isn't GPL'd).

Nor, I believe, are the WP developers claiming that Thesis contains code "copied" from WordPress, although that may depend on the exact meaning of the word "copied". At least if there are any allegations of code copying, I believe that's not the main point of contention. In other words, even if any parts of Thesis that were seen as "copied" were removed or rewritten with totally unique code, I don't think everyone would be satisfied.

The way I'm hearing it, the people on the WP side feel that, since WP is given away under the GPL, other code in the WP ecosystem (ie. themes and plugins) should be distributed under equally permissive licenses. Using a more restrictive license while benefiting from the more lenient license violates people's sense of fairness.

While I understand that, from my perspective, it's difficult to argue that plugins and themes must be open sourced, but, for example, WordPress can be used to publish content that's not distributed under a Creative Commons license. In both cases (non-GPL theme, non-CC content), somebody's profiting using software they got for free. In both cases, they're licensing their "product" more restrictively than the GPL. In neither case are they profiting by distributing somebody else's work. In neither case are they hurting the "market" for the GPL'd software.

Plugins and themes FEEL close to being "derived from" WordPress because of how tightly they work together. But just to give one more example to illustrate my point of view -- let's say somebody had published a product that was completely independent of WordPress (we'll call it CaRP Evolution :-). Now let's say one day they decided to write a WordPress plugin to act as a "shim" between it and WordPress so that WordPress users could benefit from CaRP and CaRP users could benefit from WordPress. Surely, no one would argue that CaRP would then have to be released under the GPL, right?

In case anyone says yes, consider this -- what if the WordPress plugin were written by a third party? Clearly, that person has no authority to suck CaRP into the GPL.

Now let's imagine we went back in time, and when CaRP was first developed, a WordPress plugin were developed and packaged with it -- can CaRP be non-GPL now? At most, you might argue that the plugin should be GPL'd (I'd disagree...but wouldn't care that much).

So let's apply this to Thesis. If Thesis were written in such a way that any code that directly interfaced with the WordPress API were combined into files that were separate from the files that do all the stuff that's unique to Thesis (or at least for which Chris has independently created his own implementation), even if the "shim" files should be released under the GPL, how can you argue that the WP developers should be able to exercise control over the unique parts?

Would you feel any differently if Thesis included shims for other, non-GPL blogging systems? Surely, that COULD be done. Why does it matter that it hasn't been?

At the very least, I'd have to agree with Chris that he has the right to control the licensing for the unique code he's written.