From the US embassy website:
"Is it correct that I should not lock my luggage?
In some cases screeners will have to open your baggage as part of the screening process. If your bag is unlocked then a Transport and Security Administration (TSA) screener will simply open and screen the baggage. However, if you decide to lock your checked baggage and TSA cannot open your checked baggage through other means, then the locks may have to be broken. TSA is not liable for damage caused to locked bags that must be opened for security purposes.
TSA suggests that you help prevent the need to break your locks by using a TSA recognized locking mechanism. These "special" locks can be opened by TSA using tools provided to us by the luggage industry members. You may wish to visit the TSA website www.tsa.gov for further information."
Source:
http://www.usembassy.org.uk/cons_new/faqs/faq_visa_niv.html#niv066
Chris