Not every merchant will do it, but if you recall a while back, Visa and MasterCard and retailers were involved in a lawsuit to determine whether the retailer could charge the credit card fee directly to the consumer. The ruling is that retailers can in fact pass their cost of credit card processing onto consumers. Typically this is about 3% of the balance of the transaction. Many large retailers like Wal-Mart and Target have said they will not pass the cost on to customers directly.
Regardless, the retailers cannot do it blindly, so look for signs near checkouts. Online retailers must include the fee before checkout. This applies only to Visa and MasterCard purchases, and is permissible in 40 of the 50 states. Michigan and Ohio do allow the charge. For now.
Here's a lifehacker post on the subject.