Since the ebuild don't enable gtk 3 support yet, Ibus is not working on apps that use gtk3.
Gnome team (or at least I) is willing to help test gtk3 support and gnome-shell integration.
ibus-1.3.9-r2 ibus-1.3.99.20110419-r1 in cvs. Added gtk3 local USE flag. btw, it seems that im-xim.so for gtk3 doesn't work well.