I've had this problem throughout the VB 3.0.x series. I just installed 3.0.10, with no difference.
Host is Vista/64.
Guest is Open Solaris 09/06.
Initial boot from liveboot iso image works fine. Then I click "install Solaris", and that works fine.
Then allow reboot to get the installed OS, and that is also fine. Then I shut down the Solaris system and boot again.
Thereafter, the gnome window system does not run, and all I have is the console command-line login.
Any suggestions?