Comment 1 for bug 489863

I have had the same thing happen to me. It seems to occur when I minimize Opera by clicking on the "O" on the toolbar... keeps it running in the background without showing as an open window. When I click the "O" to maximize it, Opera opens with it's address bar showing as a blank pull-down menu. That's when the whole system freezes and I have to do a hard boot. I can see that something is still going pon, since my system monitor applet still shows CPU and network activity, but the entire system is unresponsive to mouse clicks and the keyboard.
The only thing that worked once was ctrl-alt-F2, which actually dropped me out to a terminal session. I logged in and tried to startx, but it said a session was already running and I wasn't sure what to try from there.