Problem I had when upgrading from Ubuntu Jaunty 9.04 to the Karmic 9.10 beta:

When booting, I would not get the Xsplash screen, and X would not start. Instead, I would see the text boot, which would rapidly flicker.

The solution was to boot into recovery mode, and as root, run:

dpkg-reconfigure nvidia-185-kernel-source

This built the NVIDIA kernel module which the upgrade process, for some reason, decided wasn’t important enough to build itself.