I have a 2000 Dakota Quad 4x4 with the nice rims and over sized tires. Over the last two months the driver side tires have lost about 5# of air per week, it was driving me crazy - I though the valve stems were bad but could never find a leak.
I took it to the place where I bought the tires (2 years ago) and their first comment was Aluminum Rims? yes. Oh, you've got corroded rims - it was shocked, almost didn't believe them, as I've never even read about this.
Sure enough, they took the tires off the rims and the dang bead area was corroded - they used scotchbrite on a wheel and made them smooth and shiney again and now they seem to hold air just fine.
I just thought I would pass this along - they said that 60% of their slow leakers are due to this.