After having upgraded my laptop (a Dell Latitude D420) to Ubuntu Karmic Koala, it refused to boot. The normal boot process was only showing the usual splash screen, that only consists in the Ubuntu logo, then switched to a black screen, where nothing was possible anymore -- no switching to a text console, nothing. Damn.
Here's how I fixed that (in the hope that could be useful to someone). I found out, by booting without the splash screen, that the root partition wasn't mounted on boot, which was the cause of all problems. For the record, to boot without the splash screen, you have to select the kernel you want in the grub menu, edit its command-line, and remove the words "quiet splash" from it.
So I booted on the rescue kernel (by selecting it in grub), which gave me a basic busybox shell in RAM. There, I mounted manually my root partition, /dev/sda1, to a newly created directory /grumpf. I moved /grumpf/etc/fstab away and wrote a basic working fstab with the commands:
echo proc /proc proc defaults 0 0 > /grumpf/etc/fstab
echo /dev/sda1 / ext3 defaults,errors=remount-ro 0 1 >> /grumpf/etc/fstab
Then I rebooted. In the grub selection menu, I selected the regular kernel, but edited its command-line: I replaced the part root=UUID=deafbeef... by root=/dev/sda1, actually telling grub to look up the device by symbolic name instead of UUID. At this point the computer successfully booted.
Once there, I could log in as root, edit /boot/grub/menu.lst to make permanent my changes to the kernel command-line, and complete the fstab with appropriate lines for the swap, the cdrom and my /home partition. One last reboot, and voilà, the system was fully functional again.
This doesn't explain why device UUIDs aren't supported in the boot sequence on that hardware, though.