Xen dom0 in Solaris 10 
There is apparently no Xen dom0 in Solaris 10 u4 (8/07). The hope is that it will go in the next release (
ref). Judging by the release pattern so far (11/06, 8/07) we are looking at late 2007 or early 2008 for such a release.
I'm still going to download the 8/07 release and give it a look.
Also I discovered that if you select an IDE disk in the VM for the OpenSolaris-Xen image, it will start the installer. However it will die now because it thinks that the attached CDROM device is not a DVD drive, even though there is a DVD-iso attached to the device.