I read about this on the iPad magazine Project, a really awesome idea for a wallet. It's called the iWallet, although this wallet will be heavy on your wallet (pun intended) at around $600.
Basically this wallet hooks up via bluetooth to your mobile phone, and if your phone and wallet are separated by more than 20-30 feet, the wallet will sond an alarm, thus making sure you don't lose your two most important items (such a tool aren't you).
Using a biometric fingerprint reader, the wallet will only open for it's owner, so your jealous girlfriend may not be able to prove you have a card from the local gentlemen's club in your wallet. That is unless she cuts your finger off and presses it up against the reader. But hey, it's better than losing both your phone and wallet, in which case it will cost you an arm and a leg!