I have a slim box and according to the S.M.A.R.T. data on my hard drives they both hit 72C. At which point they both overheated with loud noises. I pulled the plug and left it off a couple of days. One drive has 99 bad sectors now, and the other only 1 bad sector... this is in balance with the usage. Fortunately they both are in working condition.
For Linux users I've created a cron job to protect your computer in case the hard drives are getting to HOT. I put my max limit at 48C with warnings before then. With basic usage I max at 45C. With intense usage I average 55C. Although I have pushed it to see how quickly it would go up. If I can't maintain a temp below 48 I'll have to better ventilate my computer and add more fans. Here's the script https://gist.github.com/3594541