Liz Weston in her today's personal finance post touches on the credit card issue at gas stations:
Technically, California has a law that’s supposed to prevent surcharges for plastic. But as my buddy David Lazarus has pointed out in his column, Section 1748.1 of the California Civil Code has some big fat loopholes. Gas stations get away with double pricing because they’re supposedly offering a discount for cash, not a surcharge for plastic.
DA member me1004 has more details about California gas stations in a reply in this DA forum thread