I think there is something not quite right in his answer. This is what he really says:

"I figured I'd go ahead and give an exact answer to this, despite all the talk-talk above about it.

The CC-BY-SA 3.0 Unported license is incompatible with the GPL for one simple and specific reason.

"You may not sublicense the Work."

Any given "work" rests under a license. The license is your terms by which the work-as-a-whole may be used/distributed/etc.

In our repository, we require that the license for the work-as-a-whole be compatible with the GPL. This includes *everything* in the theme. CSS, images, everything. The whole ZIP of the theme must be GPL-compatible, and by extension, everything in there must be GPL-compatible. This is *our* restriction, we want it this way. WordPress itself is free-as-in-speech and we want to ensure that everything hosted else here on WordPress.org is just as free, to everybody, forever.

So, because CC-BY-SA cannot be sublicensed, then that means it cannot be distributed in a work under the terms of the GPL. So it cannot be included in a GPL-compatible work-as-a-whole. So we don't allow it here. This is actually for your protection, as an author. If you were to include CC-BY-SA images in your theme, and then say your theme is distributed under the terms of the GPL, then *you* are actually breaking *their* license. And because we're hosting it, then so are we, by accident. Now, it's not likely to cause any stir or ruckus or anything, but if we want people to respect our licensing conditions, then it's only fair that we respect everybody else's licensing conditions as well. Therefore our rules about GPL-compatibility are hard, fast, and unbreakable, period. Doesn't matter how "free" you think the license is, if it's not GPL-compatible, then it can not be on this site."

The error he makes here, and which he actually unjustly perpetuates, is equalling 'being compatibel with GPL' with 'being GPL'. Let me explain. He begins with asserting that the CC-BY-SA isn't compatible with the GPL. The given reason is: 'because it can't be sublicensed'. But...neither can the GPL. You can't just take a work under the GPL, and then use it later on and claim it's under another licence. So, in this regard, it's EXACTLY the same. This means, the CC licence IS compatible with the GPL: if both are incorporated, then both can exist side by side, and one doesn't contradict the other.

The real and actual problem is not that they aren't compatible, but that Wordpress requires everything BEING GPL. This is the crux of the matter. You can't just change a CC (except CC0) work into something else as a licence, and you can't change a GPL licensed work into something else as licence. Therefore, if you ONLY accept a GPL licence, as wordpress does, then you've got a problem. Both licences are compatible with eachother, it's the strict insistence that only one licence can be used that makes this an issue.

For instance, if you took *the exact* same rules and terms of the GPL, yet called it differently, and gave it another name, say 'LPG'... then still wordpress would say it can't be used (and wrongly claim it's not compatible), because that licence can't be changed into GPL. Even though it is exactly the same, content wise.

The solution is rather straightforward. If Wordpress decides today that CC1 can be used as well (seen the fact that it boils down to the same), then it would immediately become 'compatible' as well. There is nothing in the 'you can't distribute it under another licence' that ain't there in the GPL too, after all. Thus, it doesn't give any MORE restrictions then the GPL, only it does it with another licence in its name.

I therefore think it is a bit misleading and FUD to claim CC-BY-SA isn't compatible with GPL. It is. The best example can be seen on wikipedia: there the GPL is used too, yet they're happy to incorporate that CC licence as well (as pictures, for instance). They don't deem it 'incompatible' at all - or they would have removed it a long time ago. The reason is simple: they allow different licences, as long as they all provide the same kind of protection/free use. The licence itself is compatible, it's the insistence no other licence can be used that makes the two incompatible.