Interesting post. I've definitely been spending some time thinking about the security consequences of virtualization. With hypervisors moving to a "free" model, from a security perspective it will ultimately come down to which architecture is less prone to attack and which vendor you trust to be working at the hypervisor level. With Microsoft making hyper-v effectively part of Windows Server 2008 and 64-bit only, it could give them an edge for Windows guests, as you don't have systems from two different vendors to patch.

Will be interesting to see how the security assessment of shared OS virtualization stacks up when its more widely used (and therefore subject to attack).