To the best of my knowledge, Ubuntu 12.04 now runs as a dom0 OS. I would not call it 'official support' because the setup isn't seamless by any stretch.
So as a side note to the original question, the Ubuntu+Xen setup was overly complicated (fragmented toolchains, spotty documentation, no well advertised FOSS GUI management tools). So we are now running off Hyper-V, which is also free and very straight forward to setup. Considering we had 0 experience in both, Hyper-V was ridiculously productive for us allowing us to focus on business rather than IT.