I've been having my Ubuntu 8.10 guest on my Ubuntu 10.04 host get stuck in a black screen. I'm running 3.2.0 PUEL w/ guest additions, more details at end.
The first time happened overnight. I didn't know to try core-dumping it per the HOWTO, and I just killed the VM (VBox itself was still operational).
The second time this happened was after attempting seamless mode. The Host+Home combo didn't work to bring up the VM menu, so I exited seamless mode, and it was all black again (host window drawing gets left there). This time, I saw the Insert Ctrl-Alt-Backspace command, and that indeed worked, kicking me out to the login screen.
Then I got on the forums and found this post, which mentions an xorg config. I looked in my guest's and it looks the same as the HOWTO, except that it's missing the line:
Could that be the source of my problems, or is it likely to be something else?