As a base, I would suggest using the pre-built image from Linaro which includes Ubuntu Oneiric as a base with all the omap-extras and boots directly into a hardware accelerated XBMC session.
Download this image and follow the given instructions to write the image to an SD-card. Booting this image will immediately give you a working Ubuntu 11.10 with XBMC running as the default session.
The default user is linaro and there is no password set. You should probably change that as your first step.