n00bs on Ubuntu posted a tutorial about installing XBMC on Ubuntu 11.04

Installing XBMC on Natty doesnt work well because there is no stable XBMC ppa for Natty (yet). There is an unstable ppa but that doesnt seem to work well either. What you can do for now is install XBMC using the ppa for Ubuntu 10.10 and change the distribution from Natty to Maverick.

  Install XBMC On Ubuntu 11.04