For the purpose of this question, I am interested in server/datacenter related hardware.

Have you had any measureable amount of ROI by swapping existing hardware to more "green" or energy efficient hardware?

For example, VMWare says you can reduce energy consumption by up to 80% by using virtualization. I have also heard of a cooling solution from HP which is suppose to reduce a small amount of engery usage (<25% I think). Google has also done something by integrating a UPS into their power supplies to reduce energy consumption.

Any real world experiences would be great, but if you have any details on initial cost, savings and pay off time about what changes were make that would fantastic. I am not only interested in virtualization, I am interested in anything.