So after my last post, I was poking around the Internet regarding RFID access control systems for buildings (for a house in my case), and I found a company called VeriChip. They offer systems that are wearable (what I originally was looking for), but also implantable inside your body, which seems pretty cool to me. It would be pretty hard to lose your house key then.

http://www.verichipcorp.com/content/solutions/1117566047