If there is water on the moon, why has it not evaporated? Is it trapped due to the lack of an atmosphere?

Here are what I think:

1. It could be trapped inside the rocks etc.
2. Gravity prevents it from going far away.
3. There was much water. The more water means that it will take longer time for everything to get evaporated, since each time water evaporates, remaining water molecules have less energy, hence the rate of evaporation decreases.

Actually the lack of atmosphere makes it harder for water to stay around, as water can not last for long in a vacuum as a liquid. It must be either solid (ice) or in gaseous form. If a gas, the molecules would relatively easily escape lunar gravity - the energy from solar particles, for example, can kick the water molecule right out of lunar gravity. So over time any gaseous water vapor won't last very long. Hence the water must stay in solid form, and to do that requires that it not be exposed to sunlight - otherwise it would sublimate to a gas. So the ice must be either buried or in permament shadow - such as in a crater at the north or south pole.

I actually just read an article about this from Digg! Earlier today, even, lol. Have a read: Moon Water: Will Lunar-Base Humans be Able to Drink It? (A Weekend Special)

It think the water is probably trapped under the rocks considering the fact that a probe had to smash through the surface to find the water.