white dust coming out of AC vents - any ideas?

One remote possibility that I doubt is the cause, but... Grab some of the dust and put it on white sheet of paper. Place another sheet over it and rub them together. Is the resulting smudge silverish? If so, your evap may be disintegrating. Have you recently cleaned the A/C or sprayed anything up into the return air duct? Your A/C is way too young for this to be a normal age issue, so if the aluminum is disintegrating, you need it checked quickly.