I would say, yes there are actual ROI benefits of “going green”. When you virtualize servers into VMware of Microsoft Virtual Server you purchase less hardware. That in it self has a great ROI. I know where I’m at now we have virtualized our Dev and QA servers (about 40 machines) into three physical servers. Two of those machines we already had, and one was a new purchase. Figure that even if we were buying the cheapest of desktops at $500 each * 40 machines is a huge savings. The there is the power savings. We only need to power those three machines now, instead of 40 machines. That leads to month over month savings.
As we move our production environment in house from our ASP to our own colo we will be virtualizing just about everything which will also lead to additional hardware and power savings.
Storage vendors such as EMC are now adding power saving features into Enterprise class storage arrays such as the ability for disks to spin down when not in use to save power. Also there are the new flash disk drives for the arrays which require a lot less power per disk (granted they are a lot more expensive to buy, so the ROI is much further out on these).