There were some other suggestions you can try, as well, based on another thread in the forums. One or a combination of these might work for you, if acpi=off doesn't. The way the problem manifested itself to me was by not seeing the disk drive devices at all. If that is the same, and the below doesn't help, try that.
all_generic_ide floppy=off irqpoll pci=nomsi
All you have to do is add the acpi=off (or any of the other options) to your kernel boot string on boot when in the grub boot loader. This allows you to test it for one time boots just to see if it works. Best choice is to highlight recovery mode, press 'e', select the kernel line, then press 'e' again.
Insert acpi=off at the end or after the root=UUID=blah-blah-blah statement, to look something like this:
kernel /boot/vmlinuz-2.6.28-11-generic root=UUID=blah-blah-blah ro quiet splash
Press [enter] to commit your change, then 'b' to boot with it.
kernel /boot/vmlinuz-2.6.28-11-generic root=UUID=blah-blah-blah acpi=off ro quiet splash
Recovery mode will allow you to see the boot process and where it could hang, but normal or single (recovery) mode would work fine to test.