I just received two Western Digital Caviar Green 1TB WD10EADS drives. There are some jumper pins on the drive that control some settings, but this one caught my eye - Power Up In Standby (PUIS). This apparently allows the drive to turn, but remain at standby rather than go to idle - so a startup power of 0.4W rather than 2.8W.
Piddly difference I know - but I might have a play with it and see what the impact is. The two drives will be a RAID 1 data partition, not used for booting the OS, so potentially they could use this PUIS feature and remain in standby until I actually need to access the drives. It's only saving ~5W from my idle power consumption (yet to be measured), but if it's not going to have a material impact on performance except for the first disk access, then I don't mind. Could also see it being a useful setting on a multi-disk NAS that might get switched on/off regularly if you have everything plugged into a powerboard like I do, allowing me to switch EVERYTHING off when I'm done with the computer for the evening.
Note that I posted this in the Green forum before jumping on me and telling me it's a dumb idea
edit - apparently WD didn't include the jumper cap for the pins, so it might be a while before I try this.