Now that's an interesting addition: low-power "green" hard drives with 6Gbps Serial ATA interfaces. A quick look at Seagate's website reveals the unlikely presence of two new Barracuda Green models, which feature 6Gbps SATA, 2TB and 1.5TB capacities, and 64MB of cache (as opposed to 32MB for the SATA 3Gbps models with the same capacities).
We've already seen 6Gbps SATA implemented in high-end 7,200-RPM desktop drives like the Barracuda XT and WD's 6Gbps Caviar Black, and it... didn't affect performance a whole lot in either case. Low-power drives therefore seem like unlikely candidates for the new standard. Seagate is the first to admit that Barracuda Green drives have 5,900-RPM spindle speeds, and it claims a sustained transfer speed for the drives of "144Mb/s"—less than half the potential of SATA 3Gbps, even if that lower-case B is a typo.
On the flip side, I suppose that huge 64MB cache and support for the new SATA standard could result in uniquely fast burst speeds. Provided Seagate doesn't overcharge for the SATA 6Gbps variants, hey, that might be good enough to justify their existence.