GreenBiz radio has an interview with Ken Brill of the Uptime Institute. Its a bit long, but all the text is there so you can speed read through it if you want.
One of the more interesting things that Brill claims is that lots of energy can be conserved by turning off equipment that isn't being used. Here's a quote from the interview transcript:
KB: Well, the institute believes that IT could radically reduce its energy consumption, and it would save money in the process. And there are a number of things that people can do and we're gonna be talking about these at our upcoming symposium. But the first thing that people can do is to go through their data center and identify what is running that could just be turned off. Between 10 and 30 percent of the energy consumption in a typical data center is for servers and storage and other things that is no longer in use. And it could just be turned — I mean, as literally, as simple as turning the switch.
Ken has some interesting things to say about blade servers too. Just like any other technology, there are important differences in blade server designs.