From the NY Times:
Every day, millions of Americans stand at store checkout counters and make a seemingly random decision: after swiping their debit card, they choose whether to punch in a code, or to sign their name.
It is a pointless distinction to most consumers, since the price is the same either way. But behind the scenes, billions of dollars are at stake.
When you sign a debit card receipt at a large retailer, the store pays your bank an average of 75 cents for every $100 spent, more than twice as much as when you punch in a four-digit code.
The difference is so large that Costco will not allow you to sign for your debit purchase in its checkout lines. Wal-Mart and Home Depot steer customers to use a PIN, the debit card norm outside the United States.