It's been pretty clear from a number of surveys, market research & industry orgs, that there are a variety of approaches to curbing IT Energy Consumption. And as you'd expect, there's somewhat of a tradeoff between complexity vs. immediacy.
First, everyone agrees that there are quick-hit actions to be taken around cooling (covering blanking panels, clearing air ductways, etc. etc.). These are super-simple, super-quick, and have a modest payback. To be sure, there are more complext/long-term cooling initiatives (e.g. air-side economizers, higher-efficiency chillers & CRAC units, etc.) but I'll skip these for the moment.
Second, there is the ballyhooed consolidation initiative. Clearly this is a high-impact approach to $ savings, but it's also going to be a year + before the full savings is achieved. (FYI, it took Sun Microsystems 12 months to complete their consolidation, and Gartner Research found that half of survey respondents expected that a consolidation project would take between 7-18 months). Also, assume that once you make the investment, it will take a year to reduce your server count. And, because most hardware is on a 3-4 year depreciation schedule, it could be as long as ~ 4 years or more before those machine expenses come off the books, and the project has complete payback.
Third, and the quickest-hit approach, is simple server power management. Implementing intelligent server shutdown (based on schedule, load, etc.) takes only a few months at most, and doesn't involve any SW configuration changes. Payback can be < 1 year. Granted, you don't remove any machines, but the power savings are essentially immediate -- and if you still want to go ahead and do a consolidation project later, you can continue to power-manage the consolidated machines.
I expect that we'll be seeing more from DOE, EPA/EnergyStar, Green Grid & Uptime Institute on these tradeoffs -- between impact/complexity, and between efficient equipment and efficient operation of that equipment.