Effective yesterday, pursuant to the terms of a $7.2 billion settlement reached last summer between credit card companies and merchants, merchants can now impose a surcharge on customers paying by credit card.

How big a surcharge depends on how much the merchant pays in processing fees, but the amount legally permissible will be between 1.5 percent and 4 percent of your purchase price.

By law, merchants intending to pass the cost along will have to post notices at check out informing consumers of the extra charge. Online merchants will have to post a similar notice to their home page.
Ten states prohibit credit card surcharges, but Minnesota is not one of them. 

You won't have to worry about being penalized in California, Colorado, Connecticut, Florida, Kansas, Maine, Massachusetts, New York, Oklahoma or Texas. 

http://kstp.com/news/stories/S2910592.shtml?cat=1
http://abcnews.go.com/Business/credit-card-fees-effect-sunday/story?id=18316310