On upgrading to Ubuntu 8.10 I had the same problem as just about everybody else: the automatic migration to the new nvidia graphics drivers involves compiling the nvidia kernel module from source using DKMS, which requires the linux headers appropriate for the linux kernel (normally linux-generic) that's running. In my case linux-headers-generic didn't get installed, so I was left without a working X server.
In the end, I think (not entirely sure how it got fixed), running nvidia-config from the command line gave me enough of an xorg.conf file to log in using 'low graphics' mode, from where I could manually install linux-headers-generic and open System > Administration > Hardware Drivers to install the nvidia driver.
Still waiting for the realtime kernel...