Latest releases of Fedora and Ubuntu are now shipped with the brand new GTK+ 2.12. In order to achieve the so-called macmenubar behaviour (the menubar of the application can be detached), you need to apply a patch against the vanilla or the customized GTK+ source tree of your distribution.
There are some Gutsy guys who updated the menubar patch for GTK 2.12.0 (gtk+-2.12.0-r2-gtkmenubar.diff.gz) but unfortunetly that cannot be applied against version 2.12.1 in your Fedora box. I feel my workspace useless without having this feature (not talking about that graphical terminal emulators block most of the GNU keybindings in shell), hence I fixed the patch so that now it can be applied smoothly against GTK+ version 2.12.1.
For getting things quickly done, download the source package of GTK+ 2.12.1, save the gtk+-2.12.1-gtkmenubar.patch at directory
/usr/src/redhat/SOURCES, update file
gtk2.spec according to the new patch, and run command
rpmbuild -ba gtk2.spec --target i686. After the compilation process finished, re-install GTK2 using the freshly created RPMs. You may also need the new version of macmenu applet. Got things done.