On a CNet article about new effort of Sun to support PostgreSQL in the near future, Martin LaMonica writes about Sun plans to integrate Xen in Solaris 10 (and obviously OpenSolaris) for September of next year.
If so Solaris 10 is going to become a very powerful platform for virtualization counting at the same time on Sun partition technology, Solaris Zones, and XenSource paravirtualization.
I also need to underline that Solaris 10 is supported on every x86 CPU and will surely support AMD and Intel virtualization extensions, eventually permitting to run Windows virtual machines inside Xen.
At no cost even for commercial use, and with this bright future, Solaris 10 could become the MUST platform for virtualization in the enterprise.