Well, let's think. It wouldn't be a water pump or thermostat problem, as once the thermostat is open, it stays open, so any nonsense about it being stuck open and causing overcooling is just that. The water pump spins with the T-belt, so that's engine-controlled and it can't be the problem, unless it's the wrong water pump for your engine and somebody managed to get it to fit. Then perhaps it circulates coolant too quickly (remember there is a gearing issue with belt-driven water pumps), however this seems really, really unlikely.
As for the coolant temp sensor, it depends which one you mean. There are 2 temp sensors, one for the ECU and one for the gauge. For the symptoms he described to be all happening at the same time (low temp reading on gauge AND no radiator fan turn-on), both gauges would have to be malfunctioning, which is unlikely, however given that overcooling is an unlikely situation in the first place, it seems possible (and definately more possible than the water pump idea).
civicwagonsonly, are you experiencing any problem you would otherwise associate with overheating, like perhaps burning coolant out the tailpipe from a busted head gasket? Maybe there's blockage at your upper radiator hose. But I would guess that when you flushed the system this would have been discovered.
Man, that's a doozy.