Can you feel any air being directed at the windshield when the A/C is on? As already stated by another poster, the blend door that directs the flow of air may not be working completely, and is allowing some air to be directed towards the windshield. This cool air being directed at the inside of the windshield glass, with outside warm air, causes the glass to condinsate.
If you've ever had a dash apart, or have removed the center radio bezel, you may have noticed the small strips of foam that are used to seal air vents or door flaps. This foam gets dry, and will literally flake apart when touched. This will obviously leave behind a poor seal. So if this is the case with one of your flaps that closes or directs the flow of air to a particular vent or vents (based on the switch in the lower center of your dash, below the radio), you'd have air "leaking" into another area of the ductwork.