Matt - it depends on what temp you need to be able to boot and how much mass your cold plate has. However, I didn't mean to imply 30 mins or anything, lol

Titch's last question I
think was concerning the issue of the automated
safety shutdown if the temps start climbing (if there is a pelt or pump failure).
Titch - I don't remember seeing one off the top of my head. The things I remember seeing on the web are all based on purchased sub-components (usually like a digidoc). Can't say I've ever actually gone looking for a circuit though, as I build and
design all my own stuff
If I run across anything in the near term along the lines you're interested in though, I'll let you know. I suspect there must be some simple cut-off circuits out there (on the web).