A coworker upgraded his OpenSolaris from snv_106 to snv_109. Upon rebooting, it failed to launch into X. For awhile, we assumed there was a problem in the grub menu.lst, but it seems the answer was a bit different.
When running OpenSolaris inside VirtualBox, you usually 'Install Guest Additions' so that you can run full-screen. Apparently, the upgrade from snv_106 to snv_109 broke the compatibility with installed guest additions.
The final solution was to uninstall and reinstall the guest additions.
From the menu (or CTRL-Home if you are already full screen) go to Device | Install Guest Additions. That will mount the virtual cdrom.
# cd /cdrom
# pfexec pkgrm SUNWvboxguest
# pfexec pkgadd -d SUNWvboxguest
# pfexec reboot
It should all work now :)