User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.4) Gecko/2008111318 Ubuntu/8.10 (intrepid) Firefox/3.0.4 Pressing Ctrl-Alt ungrabs the mouse, but only if you use left Ctrl and left Alt. Using right Ctrl, right Alt will not work nor will left Ctrl, right Alt. Reproducible: Always
What version of virt-manager, and gtk-vnc are you using ?
virt-manager v. 0.5.4-2ubuntu1 gtk-vnc v. 0.3.7-0ubuntu2
Ok, the 0.3.8 release of GTK-VNC ought to have fixed this problem. You should file a Bug against Ubuntu gtk-vnc packages to get them updated.
This bug can probably be closed then.