I think all you guys claiming heat is a primary killer of hard drives are wrong. There are tons of people keeping tons of hard drives in hot cases with totally inadequate cooling, and you just don't hear about it killing hard drives. The only heat-related drive failure I can think of is all those old stories of 1st-gen Cheetah hotswap cages bursting into flames from overheating ... and I'm going to go out on a limb and say those incidents have been exaggerated.
For 5 years I had three Quantum 10K drives and two IBM 10K drives all stacked up in a case with no venitilation across the drives and shut in a desk drawer. You could hear the drives constantly doing their little ticking thermal recalibration thing. None of them ever died or got any bad sectors. I've also had Maxtor and Hitachi drives die in a month when they were running as a single drive in a well-cooled system. Obviously Quantum/Maxtor and IBM/Hitachi had higher quality 10K SCSI drives than the more recent IDE drives products I'm talking about dying on me, but I've still never seen a drive die which I would even begin to suspect had anything to do with heat, and I've seen a lot of drives die.
I've had multiple drives from every brand die on me except Fujitsu and Samsung, but that's just because I haven't gotten many of them.
I think the most common ways for a drive to die are:
1) the drive gets knocked around and causes a head crash, either killing it right then or giving it tons of bad sectors
2) the click of death, where the head keeps seeking and banging against the stop. I've heard people say this is usually because of the sensor dying which tells the drive when the head has reached the first track and lets it calibrate itself, but I haven't really seen any fully credible explanation.
3) the reliability of the drive starts to degrade, bad sectors start slowly popping up, sometimes files can't be read, sometimes Windows gives you the "delayed write failed," etc, until the drive totally dies or you just stop using it because it's unreliable. This could be because of manufacturing defects, poor tolerances, defective materials, excessive heat, etc. Most of the time this can be considered dying of natural causes.
4) ESD or possibly heat or even just age damages the electonics. Usually the drive spins down and never spins up again. Sometimes the drive just gets totally flaky. I've seen the BIOS report a scrambled ID string instead of the actual name during POST half the time the machine boots up when the drive is close to dying.