Cammbodia is cooler (relatively) and drier in December than other times of the year - that's why it is peak tourist season.
I spent a week in SR last December (2011) and didn't experience any rain.
I didn't need to wear anything warmer than a short sleeved t-shirt and cotton trousers.
Lovely in December, you might need a light jacket if you are on a moto at night.
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