I recently installed 12.04 on an HP Spectre XT ultrabook. I installed 12.04 from a USB. When the install was complete, and the computer restarted, I got a blank screen. After some searching, I found ...
Before the upgrade, booting did not work out of the box, but this site told me to put "acpi=noirq" before "quiet splash" in the commands befre startup, and edit /etc/default/grub too. This worked ...