When rebooting after a 100% completed Jaunty-Karmic upgrade, I got a black screen after the splash. None of the hints and tips on the web worked, including returning to older kernels. Analyzing the kernels at /boot I noticed that there was a NEWER - and unused - kernel. So, I replaced the old 2.6.28-15 kernel with the new 2.6.31-14 in grub's menu, rebooted and voilá!: everything is working just fine now.
HTH,
Pipoca
S. Paulo - Brazil
HTH,
Pipoca
S. Paulo - Brazil