One idea I've always liked for solving the "unlimited" problem and complaints about overage charges is to provide fixed data transfer but do exponential throttling: the first X GB are unthrottled, the next X GB throttled to half the advertised bandwidth, then quarter, 1/8, 1/16, ... so customers can never go over the limit despite how hard they try, but they can still try to use as much as they want.

Another alternative would be constant, but very low-bandwidth, "unlimited" plans.