Is it possible - setting thermostat based on a separate thermometer
You're referring to a differential thermostat control, which I believe is fairly standard on solar water heaters:


  In active systems using pumps,
  whenever the collector is hotter than
  the storage tank, the pump should be
  on and the system circulating. When
  the tank is hotter than the collector,
  the pump should be off. This function
  is performed by either a differential
  thermostat control system or the use
  of a PV-powered pump. The differential
  thermostat controller compares heat
  sensor readings from the storage tank
  and collectors and switches the pump
  accordingly.


http://homepower.com/basics/hotwater/#ControlsComponent