I am not a lawyer, but when code is GPL, and it's distributed to you, you are given all the rights of the original author! You can change it, you can use it, you can even sell it to anyone you want for any price. You can also sue someone for violating your copyright. You must distribute the source and pass these rights on to anyone whom you distribute the code to.

As for themes, HTML/CSS is not programming code. The design is not linked to the WordPress, unless MAYBE you used inline PHP in the HTML. However, the CSS file would still be separate. A theme framework for example is GPL code. The CSS file it uses is not.

You could for example, acquire a GPL copy of Thesis from your friend, create an original CSS file for it and then sell thesis on your own website under a different name.

Any further clarity required regarding this could (and should) be requested from the GNU License team (which I am not associated with).

Here is a link to one more good read http://weblogtoolscollection.com/archives/2008/12/19/2hr-interview-with-matt-mullenweg/

Here is what I think Matt should do. Go and find another big company, say Template Monster, with lots of legal and monetary resources.

Buy their top selling 20 WordPress themes, then put them up on WordPress themes as a free download.

I am preety sure that TM will send him a C&D and then Matt and them can figure it all out in the courts.

Then *finally* we can put this unresolved issue of GPL derivative works to rest and all move on :)

Heck, I'll even have a whip round for Matt to buy the TM WP themes to start with ;)

Take a look at this post http://wordpress.org/support/topic/113358#post-543507

Matt says there

... Themes link and use lots of internal WordPress functions, which make them linked under the GPL and subject to being a GPL-compatible license.

From this, Matt's opinion is clear, themes are GPL and the recent removal of 200 themes from wordpress.org (another useful read http://www.alistercameron.com/2008/12/14/matt-mullenweg-wordpress-gpl/ ) only proves it.

Here is a another quote from matt again, from just a few days ago:

... Themes whose sites said you couldn’t modify them (which is a violation of the GPL), ...

Um, I don't really think so.

1. That templates are GPL is not legal fact, tested in US court... its the opinion (based on his legal advice) of Matt.

2. If an end user violated the GIVEN license of a theme, that theme vendor could sue..

3. Thus, an end user would be violating a given license (e.g. CC) in the hope that a court case would get judged in Matt's favor... meanwhile, that end user would be burning through legal dollars.

I think there are also legal implications for you to be dispensing legal advice... I think the popular phrase is....

.....IANAL

The license says quite clearly:

You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.

which to me means that if you just use the function, the code becomes GPL. As for the Apache argument, ASF and GPL are compatible so there is no argue there. Problem arrises when you try to use non-compatbile license like CC which brings me to Barrie.

Barrie: You are free to enforce GPL, you do not have to wait for Matt to do it. GPL gives you that right. If you download GPL code you are free to modify it as you wish, distribute it further, resell etc.

"You are free to remove those links from the theme that author tried to forbid you from (even from sponsored themes). Even when an author slaps a CC (Creative Commons) license on the theme, and demands you respect the links or work, you do not have to as they are actually violating the GPL by putting CC over it."

This is not true and I would advise against any of your blog readers to do this.

If WordPress.org say the themes are GPL, then THEY (specifically Matt as I think he is the copyright holder) are the ones that must take the legal action.

If the end user removes the links then THEY are the ones violating the copyright that theme was shipped with and could be liable.

Your claim:

This means that whenever a theme or a plugin uses just one function from the WordPress code, GPL license automatically 'kicks in' and takes over.

...is incorrect. If the author is making function calls to the core of WordPress, they can still license their code under a more restrictive license. It is only if they ship a, for lack of better words, modified duplicate of a function that's already in the WordPress core, which is when the GPL applies.

Take, for example, the Apache webserver. It is licensed under the ASF, which is more restrictive than the GPL. However, Apache makes all kinds of Linux system function calls, and Linux itself is GPL. This isn't the exact same scenario as custom themes under WordPress, but it's pretty close. Were Apache to include custom code that was derived from Linux, it would have to be GPL, but it doesn't, so it doesn't. Same thing in this case.