If you want some one to work on a Glade interface, I can free up some time this weekend to work on one. I'd probably need to coordinate closely with pipeline to make sure I interface with his files well, and I'd have to figure out the whole sudoers thing. But after that, it should just be tree lists.

Really, you could do it without glade. I'd say start out with a tree view populated with each mime type (maybe a button at the bottom to switch between file and url?). Off of the main menu would be options to add/modify the mime types, and an option to modify the apps that launch. Double tapping on an mime type in the tree view pops up a dialog box to change that type's handler (OK, Cancel). At the bottom of that first screen, and on the menu, I'd have options to save and to reset. And closing without saving will prompt "Do you want to save."

Like I said, if you want some one to work on coding it, I'll free up some time. If qole or pipeline is already working on it though, I'll leave it to them.