umm don't use your A/C ?
seriously though, i would assume it's condensation due to the humidity in the air, and how cold your A/C is getting.. does it do it ALL the time or only on really hot/humid days ? only thing i know to fix that would be to insulate the ducts better. not an easy task i'd assume.
i couldn't tell you if mine does this, i haven't had the A/C on as of yet.. windows down, radio off, blowin in the wind... even when it was 94 degrees and 70% humidity the other day.
hmm i'm not much help.. sorry..