Machine-readable passports (MRP) - Starting October 26, 2004, visa waiver travelers from ALL 27 Visa Waiver Program countries must present either a machine-readable passport or a U.S. visa. In the interest of facilitating travel, the Department of Homeland Security’s Customs and Border Protection Bureau (CBP) has given officials at ports of entry the discretionary authority to grant one-time exemptions on a case-by-case basis to certain VWP travelers without a visa or MRP, who are nationals of twenty-two (22) of twenty-seven (27) VWP countries.
http://travel.state.gov/visa/temp/without/without_1990.html