Be careful if you update to RHEL6.4 and you are using NX. RHEL6.4 removes the keymap.dir and this breaks NX at the moment.
To fix this, simply do a touch /usr/share/X11/xkb/keymap.dir. It looks like that RedHat will not fix this, because the file layout of xkeyboard-config is not part of the fixed ABI/API. The Centos guys are aware of this, and I hope they will fix this instead.
More infos:
Centos
FreeDesktop
RedHat BZ