I suppose there are caveats. If you are direct die cooling or using something like the nanoblock (if you don't already have one I guess you aren't getting one) then you don't have much buffer there like with larger copper blocks. In those cases, the pump failure might quickly kill a CPU. If you are doing something extreme like that then you might want to make a flow sensor like this:
http://www.overclockers.com/tips642/
Then connect it to a NO relay (normally open) that is suitably low in power (they make 5V relays I am not sure about 3.3). Have the relay turn off the power_good line of the CPU in the event flow stops. Might take a bit of hunting for the appropriate relay but other than that it should be cake.