If there's a will there's a way. In fact, keyless cars are easier to steal, not harder. It's not hard to hack a key fob and reprogram one to mimic another so that all one has to do is then walk up to the open and drive off with it.
You ever hear the salesman say 'hey if your keyless fob stops working just hold it close to this spot and you'll be able to start your car'. Yeah...that's called a bypass module. All the keyless cars have them. And all the car thiefs know how to use them. They don't need your fob to steal your car. Case in point - if you drive a keyless BMW you're car is a joke to steal. Using the on-board diagnostics, all a thief needs to do is get in the car, plug into the OBD, punch in a few numbers, and voila, they've just made themselves a new key to the car.
http://autos.aol.com/article/keyless-bmw-theft/
Also, to put it in another perspective, these keyless fobs basically operate in the same fashion as your garage door opener. You know how you have to hold down buttons on your garage remote to sync up with your car's homelink so you can use your car to open the garage door instead of the remote? Yea...same difference. Exact same procedure can be done with your key fob. And THAT's how easy is to steal a keyless car.