The proper UI is to show a pop-up menu (in your app's prefs) of
applications that handle this scheme, with the current default one
selected, and let the user pick. That's what Safari, iChat, Mail,
etc. all do.
In theory, that's how it's supposed to happen. Then there's the Finder
and iCal, in which case overriding them requires a tug of war using
Launch Services.
