Yeah...this has been a concern for quite a few years now, ever since RFID came out in credit cards and passports. ThinkGeek has some interesting wallets to protect against this...
Wallets:
http://www.thinkgeek.com/homeoffice/gear/c5ee/
http://www.thinkgeek.com/gadgets/security/8cdd/
Passport sleeves:
http://www.thinkgeek.com/gadgets/security/910f/
http://www.thinkgeek.com/homeoffice/gear/a7a2/
Do these work? Well I haven't personally tested them, but I'm sure a quick google search would provide some test results.