Under the terms of the Visa and MasterCard consent agreements, once approved by the Judge, merchants who accept those brands of cards will be able to direct customers toward other forms of payment (such as cash, check, debit or Discover card) without the threat of fines or other retribution by the biggest card networks.
via How the DOJ Visa/Mastercard settlement impacts your company | Shop.org Blog.
I think retailers should add a 4% markup for worst case purchase card type which would be AmEx. Then based on the swipe rates you can negotiate, I say offer discounts at checkout. 2% discount with V/MC (cc or debit), Discover, and Checks. Then perhaps a 4% discount with cash. I lumped the 2% togethre because checks can have NSF or fraud issues, while swipe fee for Visa/MC CC or debit, seems very similar especially for micro-merchants. Cash, as long as you verify big bills are legitimate, is secure and immediate payment.