In 14 trips I have never had a merchant charge me extra for using a credit card - at times I have offered to let them add it and still have never had it added.
Leah Ann, that isn't the deal. The deal is that they give you a big discount (often 15%) if you pay cash. That way they don't have to identify that there is a surcharge if you charge. I am sure that this doesn't apply to restaurants or hotels.