I have a homebuilt WHS that I have been using for about 8 months or so. I reused several PATA drives that I had been using in various PC PATA raids - 4x160 GB ATA 133s, and a WD 400 that was a couple of years old. The final 160 GB has failed today, the WD 400 failed a couple of weeks ago, the other 160GBs along the way. What I think is going on is that when the drives were in the PC they really weren't being used much overall. They served more as occasionally accessed storage. In the WHS environment, the drives are being used much more often - the balancing, backups, streaming video, copying new media to the WHS, copying it off the WHS, etc. In other words, drives that spend most of thier lifetime with the heads parked had to start working, weaknesses were exposed, and failure resulted. The problem is not heat related, the WHS is a 4U server case with boocoo cooling fans.
My supposition is that the WHS environment is not a good one for reusing old HDDs, both because it is a 24/7 environment, and the activity that the drives will endure.
I currently have a new 500GB and a fairly new 320 GB in place now, and plan on adding at least another new 500 asap. I will not rotate older HDDs to the WHS from now on.