I entered "sudo" into the verve plugin by fault. As a consequence the whole desktop environment got freezed. I was able to move the mouse, but I got no response from any application. If I switch to a text-mode terminal and back to the X11 display, the content of the display is a little bit scrambled. I can get out of this lock if I kill the process xfwm. After this, the keyboard get available again (but all windows got closed). I could reproduce this bug on two different machines (both x86-64) with Xfce 4.4. Of course, it is nonsense to use "sudo" in the Verve - this was my fault. But it should not lock the whole desktop environment...
This is really not a problem with xfwm4. this is a bug as old as X itself. If you run a interactive command that requires user input without a console, X freezes. You can try with any window manager you want, that'll be the same.