Originally Posted by BluEyes

biopete: radiator fans being on all the time is another problem (and could lead to the fans burning out and the car overheating in the summer) but would not cause overcooling with a properly functioning 'stat. See, when the stat closes, it bypasses the coolant back to the block, so it does not have a chance to get to the radiator and get cooled by the fans.
Also, then you're on the highway there is more air flowing through that radiator than the fans could ever hope to pull, and the engine should keep temp just fine.
If your car doesn't get up to temp, or takes a really long time getting there, you need a new thermostat. At least they are cheap to buy

Now, if you have manual heater controlls and have the heat and fan on full all the time, that could cause overcooling because the 'stat cannot regulate flow through the heater core.