The latest version of Gtk2hs, namely 0.12, supports GtkBuilder. Relatedly, the latest version of Glade is buggy and refuses to read libglade-format files. So I can't edit the user interface any more. Sadly, Ubuntu Natty does not have Gtk2hs 0.12. Oneiric does. I think it's worth supporting Natty, so sadly GtkBuilder-ification will have to wait.
Fixed in Git for 0.4.1.
Use of freedesktop.org services, including Bugzilla, is subject to our Code of Conduct. How we collect and use information is described in our Privacy Policy.