I've finally hooked up bzr-dbus to bzr-gtk: When a local branch changes its HEAD, that notifies a background task 'commit-notifier' (when the branch can be read by that task). You need to be running 'bzr commit-notify' and have bzr-dbus correctly installed (see its README). Its only just working now, so will be tweaked and tuned a bit to have the commit-notify command started automatically by GNOME, have it read remote branches (not done right now to avoid issues with needing a login box).
One nice thing is that this will notify on 'pull' commands too, so when a bzr branch or pull command completes, you get a notification.