I've only seen 2 gas stations do this and they've been doing it since gas went north of the $3 mark. Both stations in not-so-great areas of Tampa, FL.

Note: Gas stations aren't allowed to charge higher prices to credit card users, but they are allowed to give a "cash discount". Depending on the state you live in, the station might be required to post the highest price on the road sign. In other states (Florida), as long as they have "cash price" on the road sign, they can post the cheaper prices.