From the Phoronix article:
From the Xen release notes:With Xen 4.1, the Xen dom0 and guest support is also available in most upstream Linux distributions to work unmodified. The dom0 support is with the vanilla Linux 2.6.38 kernel and then the rewritten Xen PV-on-HVM drivers were tacked on one release earlier, the Linux 2.6.37 kernel.
dom0 suppost is pointless if you can't boot any VMs AFAIK, we're looking at 2.6.40 at the earliest, as some of the needed parts are not going to make it into 2.6.39.Basic dom0 support was added to the Linux kernel and a vanilla 2.6.38 kernel is now able to boot on Xen as initial domain. There is still some work to do as the initial domain is not yet able to start any VMs, but this and other improvements have already been submitted to the kernel community or will be soon.