The Stealth Case for iPhone 4/4S from HEX incorporates a compartment that holds an RFID-enabled card to make touch-to-pay available with a touch of your iPhone case. The Stealth is made of polycarbonate and is available in either black or white with a matte finish. Inside is a compartment to hold an RFID-enabled card; shielding prevents the card and phone from interfering with each other. If you don’t have an RFID card, you can use the compartment to hold another card or cash. The Stealth is available now from HEX or from Apple for $34.95.
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Or.. You can embed your RFID card in your iPhone case….
http://hackaday.com/2012/07/17/store-your-rfid-transit-card-inside-your-cellphone/