Happy Melon - I should have clarified how I thought this should be implemented. I don't mean that any extensions should move locations - the fact that everything is in one big /extensions directory is quite helpful. I just mean that there should be some SVN settings so that only users with a certain permission level can modify the code in pre-specified directories under /extensions. I don't know much about SVN administration, but I assume that you can restrict access in that sort of semi-fine-grained way.
-Yaron