"Thermal switch" ??
That's me !!!
About a year ago, I proposed this solution:
The green wire carries the "PS_OK" signal, which the mobo will hold to ground as long as it senses that all voltages are OK. If something goes wrong (CPU cove voltage drops, for example), the "PS_OK" signal is released from ground (floating), and the PSU immediatly shuts down.
I studied the ATX spec that I downloaded from here:
http://www.intel.com/design/motherbd/atx.htm
Then I purchased a thermal switch from there:
www.MECI.com
I hoked the thing together, and it worked perfect (I tested it by applying the thermal switch against a soldering iron).
I came up with this protection AFTER I experience a meltdown. :-(
This is the simplest system. I consider it safer that software protection. If you set your core clock to close to the stability limit, then the conputer will crash, preventing any software protection to act. You can still back off to make sure it won't crash (back off ??? NO WAY ;-) ).
The best fail safe design, in my opinion, is a system that will read the temp of the CPU (I think the staff here has tested that) and will shut down the PSU (can be done with a relay on the green wire) when the CPU core temp exceeds a define threshold. The "Thermal switch" design only senses the water block. Better than nothing.
Hope this was helpfull.