the uim ebuild's postinst does 'gtk-query-immodules-2.0 > ${ROOT}/etc/gtk-2.0/gtk.immodules', but it seems like gtk under amd64 is expecting the immodules file to live in /etc/gtk-2.0/x86_64-pc-linux-gnu/, so uim doesn't come up as an option until the gtk.immodules in this directory is updated.
Fixed in cvs (0.4.6-r1).