This falls into the "so obvious we all overlooked it" category of technology innovation.
While tech journos including yours truly have been falling all overselves in coverage of power management and coverage of virtualization as SEPARATE topics in the green space, we seem to have forgotten the fact that these two topics actually need to be entwined. What good is a power management strategy that considers JUST the hardware, after all? We need something that accounts for vitally important virtual machines.
Eaton's NetWatch 5.0 and Network Shutdown Module 3.0 were designed with this problem in mind, and both were recently tested compatible with VMware ESXi 4.0. The applications handle the shutdown of virtual machines and hosts connected to Eaton uninterruptible power supplies (UPSes). Mainly, this is in case of sudden outages, but I can envision uses within organizations that are trying to more proactively managing certain servers that can afford a little downtime for the sake of reduced power consumption. Here's a link to all of Eaton's data center-relevant power management software.