Yeah, they can get toasty. Whether actual burns would be realized, I dunno, but they can get very warm.
The warmer the drive, the shorter the MTBF = Mean Time Between Failure. A cool drive is a happy drive, and you do want your drive to be happy don't you?
In my praetorian case, I have my raptor and two 7200/8mb maxtors in RAID0 mounted in the drive bay right behind my two front intake panaflo L1A's. They are quiet and give plenty of good airflow to keep the drives reasonably cool. I expect them to last me quite some time into the futurem, but if they were running hot, I would do backups much more frequently.
I am feeling the sides of them and they are pretty darn hot. I have both sides off my case and the air from my airconditioner passes over that direction however lightly. I didnt think drives got that warm. Is there a way to check them? I got them at different times, they couldnt possibly both be bad right?