I was pretty happy to see that Ted Samson of Infoworld wrote a really well-balanced analysis of the advantages (and calculated risks) of using server power management in the data center.
Besides speaking with leading SW companies in the space like Cassatt, he also pinged authorities at HP, IBM and Sun, who all had thoughtful positions on the merits of powering servers on/off based on their usage (and when they were idle). He also spoke with Robert Aldrich of Cisco Systems -- who pointed out that they are already using power management quite extensively internally, with no ill-effects. BTW, Robert pointed out that servers at Cisco use 40% of their power when idle. Most analyses I've seen show that the number is closer to 60%-80%. Good for them.
What's also fascinating about Ted's post are the comments. Most are quite supportive of power management, with a realization that future, denser data centers will need this -- and that some power companies have already figured-out that dynamic "load management" is one of the most intelligent operational innovations available today.