With Rob McQueen's help, I cooked up a patch to gaim that makes gaim open urls in the GNOME web browser. Gaim will now detect if you are running GNOME and automatically open URLs with your GNOME web browser. No extra configuration is required and the browser section of the preferences tab is actually hidden if you're running GNOME.
The patch against 0.66 is here. libgnome 184.108.40.206 or later is required since this uses the gnome-open binary.