Basically, beacuse the engine is only idling, the water pump is only pumping water around the engines at IDLING speed, so the temperature increases.
Water pumps are at their maximum output at around 3000-4000rpm in a car, so as soon as you get going, water is flowing around your engine faster, thus making it cooler - Which is why your temperature guage drops!
And yes, if it goes above the notch after halfway, then I'd say replace the thermostat, otherwise you could encounter serious problems.
Ian.