Cabin air filter is definitely the problem, as mentioned numerous times before. However under certain circumstances the cabin air filter can temporarily get a mildewy smell that is not permanent but merely due to extenuating circumstances. For example in particularly rainy and humid weather (i.e. Virginia summers and springs), if the car has been sitting for a few days outside the filter can get a little moisture in it -- causing the mildew smell. This is not permanent but goes away after driving the car for a bit.

I mention this just so people double check before throwing out a good cabin air filter!