Well if it's a MediaWiki extension, it has to be GPL-compatible, otherwise using it as part of MediaWiki violates the core's own GPL license.
Wrong. WMF can use any software they like on their servers... even propriatary software. They are _using_ it, not _distributing_ it.
Compatible licencing is only relevant on software that is distributed, in the WMF's case, MediaWiki and related extensions.
I wasn't even aware though that extensions to be distributed needed to be licensed under something that is GPL compatible. It's been a while since I read the GPL.
Looking back over it again (well the FAQ actually), that is very non-intuitive... we'd need to fork the Mediawiki process to allow non-GPL extensions to be distributed?
I might have to look into licenses again and make sure what I use is GPL compatible. The GPL is such a pain sometimes....
Thank you, Derric Atzrott