Interesting...
I had a similar thing occur with a Western Digital Caviar SE16 (WD5000AAKS) in my 20" Aluminum iMac. I was in the middle of some web browsing and some music in iTunes when all of a sudden the music started skipping and Safari was "beach balling" like crazy. Since I hadn't restarted in about 30 days, I figured a process went on the fritz and I was just better off restarting.
...well, it never came back up.
I tried to Target Disk the iMac ...nobody home
I pulled the drive and connected it to a SATA to USB adapter. It mounted after 15 minutes of hard disk activity, but would only allow me to copy about 500MB of data before the drive would just unmount and power down. Thank God for Time Machine....but anyway...
A month later, I had some non-critical data that I forgot I exempted from backup that I really had no desire to download again and thought I'd try and grab it 500MB at a time. Except this time the drive mounted immediately and I was able to copy 120GB of data and run 12 hours of WD diagnostics with no problems whatsoever.
I had already replaced the drive with a WD Caviar Black 1TB (love this drive BTW), but WTF?! A drive that was obviously "toast" just decides that it hasn't spent all 9 lives, 30 days later?
I had possibly figured heat/warping, but the time between pulling the drive and taking it to work for recovery on our IDE/SATA to USB adapter would have been more than enough time for the drive to cool.
Static electricity, heat, or firmware bug? 30 day sick leave?