Run following script to set uzbl as the default browser in GTK+2:

#!/bin/sh
gconftool-2 --set "/desktop/gnome/applications/browser/exec" --type string "uzbl-browser %s"
gconftool-2 --set "/desktop/gnome/url-handlers/https/command" --type string "uzbl-browser %s"
gconftool-2 --set "/desktop/gnome/url-handlers/unknown/command" --type string "uzbl-browser %s"
gconftool-2 --set "/desktop/gnome/url-handlers/http/command" --type string "uzbl-browser %s"

You can test it with following command:

perl -e 'use Gtk2; Gtk2::show_uri(undef, "http://google.com/" )'