http://www.govexec.com/story_page.cfm?articleid=33436&dcn=e_gvet
"Low-frequency RFID chips be read from up to 20 feet, but the department has maintained e-passports would include chips that only can be read from 'approximately four inches' away from the source. To eschew concerns over privacy and safety, the department said the front covers of e-passports have a built-in anti-skimming device. It is akin to 'wrapping them in tin foil to prevent the radio frequency signal from getting through,' said Jay Stanley, communications director at the ACLU's Technology and Liberty Program. "
