My datacenter reached 92% virtualization Q42011.
The "% virtualized by server" metric is something I recommend every administrator track. I have been doing this at least once per year but per quarter is probably a better frequency. I also track "% virtualized by application". We reached 90% for this metric in the same time-frame.
Note that this is a ratio of virtualized to virtualizable servers (if that wasn't a word, it is now!). I don't consider servers that can't be virtualized - for example, an HP-UX OS running on Itanium hardware. These servers aren't important because come upgrade time, your standard platform is vSphere so the rest will take care of itself over time.
Tracking these metrics will give you a good indication of progress over time. It's very interesting to see where things stood 1, 2 or 3 years ago. It would have been nice to to compare this to energy costs but unfortnately either we haven't tracked this costs or I don't have access to it.
Overall, this is a good indicator of where you're at in your virtualization journey.
A final note: that last 10% is the hardest!