good question, I've been thinking about that. The amount of heat can't be TOO high (average for a 750gb drive is 13 watts of power, so even assuming 0% efficiency it's going to be 13*32=416watts of heat max (ok, this won't ever happen but it's the absolute worse case).
I was thinking perhaps to use some type of heat-pipe around the drives to bleed the heat to a bus bar or something in the case that would then be cooled by the same water loop that's cooling everything else.
Since the drives are all hot-swap that is what is making it interesting due to the nearly zero tolerance around the drives. I _may_ be able to also fit a heat shield around the drives (aluminum place on top/bottom of drive) just to get something to equalize the temp and to help pull it to the side of each of the bays where I can get at them better.
Haven't really found a silver bullet yet.
You have any ideas?