As described in http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=531771 the fix for http://bugs.freedesktop.org/show_bug.cgi?id=19500 broke the usage of lvl3 modifiers (at least) in GTK+ applications for users of PPC Linux. This happened with the upgrade to xkb-data 1.6.
This bug has also been reported on amd64 and i386 (see debian bug reports for ex.) -> adapted title and hardware accordingly
*** This bug has been marked as a duplicate of bug 19500 ***