For the record -- sounds like you're aware of this, rjk, but I've seen people that aren't -- Xen is open source, and doesn't cost anything; that includes the hardware virtualization support.

It just so happens that XenServer uses Xen as part (a big part, I'd presume) of its commercial offering, but there's no reason regular people (who don't hate money ) can't use it also, for free.