http://www.mastercard.com/general/zero_liability.html
"Zero Liability
Have peace of mind knowing you won't be responsible for unauthorized purchases.
Receive the benefit of not being liable in the event of the unauthorized use of your U.S.-issued MasterCard. As a MasterCard cardholder, coverage extends to purchases made in a store, over the telephone, or on the Web.
Zero Liability is provided under the following conditions:
Your account is in good standing.
You have exercised reasonable care in safeguarding your card.
You have not reported two or more unauthorized events in the past 12 months.
What to do if...
If you suspect unauthorized activity on your account, stop using your card and contact the bank that issued your MasterCard card as soon as possible. "