As allowed by the new financial reform law, retailers will be allowed to impose a $10 minimum on credit card purchases. This Consumerist article
reviews how the credit card companies are changing their policies on this. Here's what Visa has recently added to its website
U.S. retailers may require a minimum purchase amount on credit card transactions. The minimum purchase amount must not exceed $10 and does not apply to transactions made with a debit card.
It's nice to see the minimums won't apply to debit cards. I don't know for sure if this applies to both PIN-based and signature-based debit card purchases.