Many rhododendrons are decidous and do lose their leaves at this time of year.

If it is a naturally evergreen type, many of the oldest leaves will be coming off about now--the ones that grew late summer of 2009. Other than that there are a variety of causes:

Watering problems: too little, too much, irregular, etc.

Lack of food, or poorly timed feeding.

Bad drainage--which should have shown up the first year the plant was in the ground.

Too much shade, or too much sun, as the angle of the sun changes, and the surrounding trees lose their leaves.

The only way to give a good diagnosis is to know your climate, the plant's exposure, and growing conditions.