Preedit has been fixed in gnome-shell entries, so ibus should make use of it. See https://bugzilla.gnome.org/show_bug.cgi?id=684453
ibus-1.4.99.20121006-2.fc18 has been submitted as an update for Fedora 18. https://admin.fedoraproject.org/updates/FEDORA-2012-15547/ibus-1.4.99.20121006-2.fc18