It depends on the ubuntu user, but I'm running a linaro pre-made image for xbmc on the pandaboard so for me X is always run as 'linaro' user. You should check yours.
I put those lines in rc.local so they are always executed on bootup. A better solution would probably be to put the corresponding settings in xorg.conf, but since I don't have an exiting one (ubuntu now defaults to not having one I believe), it was easier for me to just do it this way.