The next version of gnome is going to start the transition to gtk3. I don't expect it to be fully completed by F14, but there will certainly be applications which use gtk3 instead of gtk2.
Since gtk2 and gtk3 are parallel installable, gtk3 needs its modules to be linked against gtk3 and installed in $libdir/gtk-3.0/3.0.0/immodules instead of $libdir/gtk-2.0/2.10.0/immodules. The code itself does not need to be changed (provided you do not use deprecated apis or direct access to struct members).
I have added gtk3 support in upstream. I will build it for fedora soon.
Fixed in ibus-1.3.5-1.fc14