UNITED STATES visitors with British passports, perhaps confused by various announcements from the Department of Homeland Security (DHS), are reminded that from 26 June 2005 they must have a machine-readable passport to enter the USA. In a practical sense most people will have by now a passport meeting these requirements. Machine-readable passports have a sequence of lines that can be swiped by US Customs and Border Protection (CBP) officers to confirm the passport holder”s identity quickly and to obtain other information about the holder typically found on a passport”s inside cover. 27 countries including the UK are participating in the scheme.
http://www.usembassy.org.uk/cons_new/visa/niv/vwp.html