Identity theft is a constant concern in the digital age, and the inclusion of RFID chips in just about everything from cell phones to passports unfortunately opens up another avenue for would-be thieves to lift your data. The only way to really know you were secure would be to block all wireless frequencies from reaching your IDs and gadgets. ThinkGeek's solution is the Blokket, a small pouch designed to hold mobile devices or RFID-chipped IDs and block them from receiving any incoming wireless signals.
Made from a mesh of nylon and silver, the Blokket works like a mini Faraday cage, isolating whatever's inside from outside connections. At 8" x 5.75" (20 x 15 cm) it's large enough to hold a cell phone along with chipped credit cards, IDs, passports, and such. The fabric is said to be capable of blocking 99.7% of radio frequencies, so the odds of any shady characters snatching your data are slim to none.
The Blokket is probably more useful for IDs with RFID chips than for cell phones though, since most mobile devices can either be set to Airplane Mode or just switched off entirely. Also, leaving a phone inside the pouch while it's in active mode will probably drain the battery faster, since the phone will use extra energy searching for a signal.
ThinkGeek is currently selling the Blokket Signal Blocking Phone Pocket for US$24.99.