I've been to some shops with the Amex accepted signage, only to have them offer me a discount if I were to pay by cash or V/MC.

While I understand and empathize with trying to reduce costs, I don't think these Merchants appreciate how badly this makes them look to their clients/customers. Personally, I wouldn't feel comfortable doing business or dealing with folks who employ these sort of tactics. As to restaurants, it makes me wonder what else they're cutting corners on, if eliminating the small BPS of translation costs is that important to their operation.