The completely new monitoring plugin, which uses the VI API, is available in viewtopic.php?f=35&t=1648
This can be used with both Nagios and MRTG. It can produce stats for a particular guest or for a whole farm, so it can be used to give correct values for CPU and Memory use of a virtualised server.
Note that it works by pulling data from the VirtualCentre server, so you need to have one of these and create a read-only account for the plugin to use. It is also rather CPU-intensive thanks to the SOAP::Lite API calls.