ergophobe - 3:43 pm on Oct 5, 2010 (gmt 0)

So you can do things like writing a GPL interface to your proprietary code.

Yes but you can't write a proprietary interface to a GPL application. There is a specific exception for hooking into runtime libraries in a programming language.

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.

That's typically where people get into trouble and why some people complain that the GPL "contaminates everything it touches", which could be an issue in flood6's situation (if, for example, you were a dev shop and wanted to have proprietary "added value" extensions, they would have to be GPL).