The new drives boast a 40% reduction in power consumption compared to other models, which at 5 watts less may not seem like much but can add up quickly when factoring in thousands of drives. This puts these new drives at requiring less than 10W each, and while it's a drop in the bucket compared to processors and video cards it's still a large area of concern in datacenter environments.
The RE2-GP2 series as they are called will be available in 500GB, 750GB and 1TB capacities, with prices ranging from $150 to $350. They are aimed at the enterprise only at the moment, but it's likely that the same power reduction features will eventually carry over to their desktop line. You can read the full press release at Western Digital's site.