graeme_p - 7:38 pm on Oct 8, 2010 (gmt 0)

What I meant by that is you can put your proprietary code in a separate work (ideally run it in a separate process), and write a (GPL) extension that communicates with your proprietary code.

I suspect the GPL 33 is a bit stricter about this because it IS a loophole.

So if you want to build a custom app for internal use that extends a GPL app (like a module for a GPLed CMS), that extension has to be GPL.

Not if its purely for internal use. If it does not leave your organisation then it has not been distributed (the GPL 2 term) or conveyed (the GPL 3 term) so there is no one you have any obligations to under the GPL.