Western Digital is the next in a series of high-profile companies that is trying to gain PR by "going green". WD has joined The Green Grid, a group dedicated to improving energy efficiency in computer environments that are long thought of as power-hungry, such as businesses and datacenters. Hardware companies like WD that sign up primarily do their part by creating hardware components that use less power. While hard drives are not exactly the most power-greedy components in a system, they are certainly noticeable. Particularly with very fast drives, such as 10,000RPM drive.
They are probably using this as a method of getting more people interested in their GreenPower HDD lines, which similar to other vendors tries to create a drive with high capacity but lower power draw. They cite figures such as a drive costing $10 less per year to operate when designed for low power consumption. Not much on its own - but in a datacenter where you might have several hundred or even more, it adds up quick. You can read more in the press release.