I was reading a post on Steve Riley's blog that linked to a couple of very interesting posts by Jeff Jones on his Think Security blog. He goes into quite a bit of detail on Patchguard and also interviews Forrest Foltz who is a Patchguard Architect at Microsoft. They discuss some of the truths and misconceptions around Patchguard, the research reports outlining methods of bypassing it, mitigating factors, etc. Also very interesting were some thoughts about how hypervisor based virtualization, with the addition of patchguard like functionality to the hypervisor could go a long way toward mitigating current methods of bypassing the technology.