There's a problem between gtk 2.12 and xfwm 4.4.1-2 and before. A deadlock may occurs which renders the session completely useless. The mouse still moves, and you can Ctrl+Alt+Backspace, but nothing else works.

It's a gtk 2.12 change which is fixed in xfwm4 4.4.1-3, uploaded this morning. If you encounter this behavior, upgrade your xfwm and it should be ok. All archs aren't built yet, but buildds are keeping up.