I had trouble booting the last few kernels that came out in 11.10 (3.0.0-16 and 17), and alas the same trouble with booting 12.04 which I installed today.
My root filesystem is encrypted, and I expect during the boot process to get a prompt for the encryption password. This doesn’t happen, and instead the boot drops into busybox. I was able to just keep using 3.0.0-15 under 11.10 but now that 12.04 is installed I can’t do that anymore. Actually have to try to track this down. .
After a lot of digging around, I haven’t completely got it figured out, but I did find some hints in this tread: https://bugs.launchpad.net/ubuntu/+source/cryptsetup/+bug/874774 which gave me a start, and at least enabled me to boot. The thread describes a mixup in matching device names from udev to names in /etc/crypttab. If a match isn’t found, you don’t get a prompt for a password, the device doesn’t get luksOpened, and the boot fails waiting for it. So, the trick is to ensure the matching logic finds a match between the udev devname and the entry in /etc/crypttab. I actually didn’t even have an entry in /etc/crypttab for the root device. Silly me. I had entries for a couple of other encrypted devices. I would love to report that all I had to do was put in an entry in /etc/crypttab and it all worked. I alas not – I haven’t yet managed to get the problem solved. But I now know where to look, and more importantly reading the thread mentioned did make clear the short term workaround, which may be clear to everyone but me: it didn’t occur to me that once I dropped into busybox, I could just do cryptsetup luksOpen /dev/sde3 sde3_crypt, type in the correct password, and then exit busybox. The boot process resumes and is successful.
Update: In the normal course of Ubuntu updates I got 3.2.24 and the problem spontaneously healied itself. Now upon boot, I get an early prompt for the root password. Bob’s your uncle.