The thermostat could be stuck open, but you should still get some heat at idle. How does the temp gauge look at idle, when you have no heat?
Some other things to check/think about:
-With the engine warmed up, feel both of the heater core hoses. They should be the same temperature. If not, the heater core is plugged and needs to be flushed and backflushed. Check in the "maintenance" section, listed at the top of the page, for info on this.
-When was the last time the cooling system was flushed/filled? It could be a matter of a dirty cooling system.
-This could also be a faulty water pump. Both radiator hoses should be the same temperature with the engine at normal operating temp.
-If coolant is disappearing slowly, and you're not seeing a leak, take out the degas tank (commonly referred to as the overflow tank) and check it for leakage. Often times, the bottom of the tank will crack and leak slowly. Most times you can't see it, since the framerail is directly below the tank.
Good luck, and let us know if you need any more help!