white dust coming out of AC vents - any ideas?

Well, without seeing just what your access to the areas is like, I think I'd be tempted to say try the foil tape for ducts. I've had good luck with that over the years. If properly applied it seems to stay in place for a long time. Whether it matters or not, though, I wouldn't buy it at a surplus store. I'd try to get a "name brand" or something with good reviews.

Good Luck,

~Rick