Glynor, et al, go ahead and shoot this down, but i think the benefits of a fan are marginal. Marginal--meaning a bad drive will go quicker without a fan, but a good drive's lifespan won't be noticably enhanced by a fan. I have both, and had drives go the pooch route in both. Since I hate fan noise more than I hate buying new drives, ALL my new boxes are fanless.
I think the most important concept is avoiding WD drives. Except for an Hitachi "Deathstar" that basically fell over gasping on boot, ALL my dead drives have "WD" on their tombstones. Funny, I have yet to have a Seagate go bad. And I have Seagates in fanless boxes.
Remember, there are no iPods with fans. True, the iPod case is far superior to any 3.5" consumer box (the key is thermal contact, carrying the heat away). But I think you're fairly safe, as long as your data is backed up.