] Regarding bags that block radio signals; we (me,
] colleague who shall remain nameless) tested this with our
] building access cards and our cell phones. An anti-static
] bag for computer chips was demonstrated to NOT block the
] signals. However, an (aluminized) anti-moisture bag for
] corn chips (specifically, Fritos) successfully
] deactivated both my cell phone (GSM) and my access card,
] even pressed directly against the reader.
]
] It is a small help to know that effective RF shielding is
] available in many snack vending machines, and that the
] shield itself is a mundane enough item that it will not
] attract attention.

Pringle's cans and Fritos bags!

Aluminum-lined junk food packaging: It giveth the signal, and it taketh it away.

Freedom to Tinker: Why Use Remotely-Readable Passports?