As long as the case has good overall air flow, nothing is needed. Hdds don't get very hot.
if you can keep a finger on my WD Caviar for more than 2s without screaming, i'll be very surprised.
I have 4x 500GB WD Caviars in RAID5, all stacked on top of each other, and they heat up like CRAZZZZZZZZZZZY under load. Sometimes i think they'd melt the sata cable.
I'm looking for a cooling solution too, but these drives don't seem to be complaining, so maybe I dont need one.