One caveat, folks. Jusst because the drive is "dead" does not mean that the drive is the culprit. I learned the hard way and I'm a tech. I just figured that it was the drive and ordered a Matshita U875 (a better drive than the Sony Optiarc drive anyway).
Another warning--taking apart an aluminum iMac is not bad compared to the first gen of the Intel iMacs, but it is still NOT for the novice to attempt. There's just too many potential problems that can occur--wires to cross, connectors to break, screws to drop behind the logic board, etc.
Back to our story: The drive, after uninstalling the old drive and installing the new Matshita, and reassembling behaved the exact same way ad the OLD DRIVE!!! It is undoubtedly the I/O circuit of course built in to the logic board. Gave the bad news to the client that he's better off getting an external drive than paying extortion rates to Apple for a replacement logic board and the labor to install it. Apple execs--"Hmm...if we make the price of a replacement logic board 80% of the price of a brand new computer, we will sell MORE Macs!!"
Lazarus