Just search for blue pill hypervisor to get the background. We've talked about it here before. VMware's Beng-Hong Lim and Keith Adams weighed in. Now Thomas Ptacek and Nate Lawson have challenged Joanna Rutkowska to a cage match at this year's Black Hat Briefings conference. I think at this point the argument is more about theoretical detectability in the field, Joanna having conceded that one can always check timing, but arguing that you can't prove the negative that a virtualization-based rootkit will never be invented. I am not losing any sleep.