It would be nice to be able to chose between Gnash/Flash within the browser if both plugins are installed in the system.
This bug has been reported first by a Debian user who was trying to do so by setting the MOZ_PLUGIN_PATH environnement variable, but this variable is not relevant anymore with Webkit.
On Mac OS X, one can just put the plug-in into "~/Library/Internet Plug-Ins" directory instead of "/Library/Internet Plug-Ins".
I'd argue this should be done at the browser level, not the rendering library level. I may want different webkit browsers to do different things, and different xulrunner browsers to do different things.