After running an upgrade 'sudo apt-get upgrade' and 'sudo apt-get install xbmc' to get my system up to date (to the latest SVN) I started getting the following error:
XBMC needs hardware accelerated OpenGL rendering.
Install an appropriate graphics driver.
Please consult XBMC Wiki for supported hardware
The first time I had this happen to me, I spent a number of hours trying to figure out how to get my system back up... but by 3 am I gave up. The next day, I reran the XBMC Live installer and got my system back up. This time, I didn't want to have to redo all the changes I made, so I wanted to fix things. I did a bit of research and this time I tried to directly install the nvidia drivers right away before mucking around with my configuration.
I originally tried the following which did NOT work for me:
sudo apt-get install nvidia-185-kernel-source nvidia-glx-185 nvidia-settings mesa-utils
Then reboot with 'sudo reboot', but like I said it didn't fix my issue... so I ran the NVIDIA installer directly as noted in this thread:
sudo apt-get remove xbmc-live sudo nvidia-installer --update -a -s -f sudo apt-get update sudo apt-get install xbmc-live sudo reboot
After a reboot, my system came back up and worked like a champ with the latest version of XBMC (and the NVIDIA drivers) installed.