The EPA is doing a bang up job on the new EnergyStar standard which will include servers as well. A lot of added gains in efficiency need to be attacked at the design and material levels, and IBM and Intel are doing just that with new materials like high-k; sounds like a breakfast cereal. EnergyStar is supporting that work but is also addressing the macro level such as the fact that half to two thirds of your servers' costs are hidden away in power and cooling.
Unbelievably, only half of the juice ever even makes it to the CPU! It's a one to one relationship - one kilowatt for the server, one kilowatt to cool it. Per kilowatt, that translates into $12,000 minimum for upstream hardware and another $3500 for site costs. It's hard to win the battle when you don't know the casualties count, but EnergyStar is definitely helping out with that.