The Exchange team has published a whitepaper on TechNet comparing the power utilization of a typical 8 server physical Exchange environment compared to a a virtualized environment using the same number of logical processors but on only two Hyper-V host servers. The result: a 50% reduction in power usage (excluding storage). If storage is included it was a 37% reduction.
The purpose of the test was to isolate on power consumption scenarios so other things like cooling and space savings weren’t considered but would also be measurable benefits. There is also a link to general guidance on virtualizing Exchange as it makes sense in a lot of scenarios but not all.