Does your car have remote keyless entry? This may come in handy someday. Good reason to own a cell phone.
If you lock your keys in the car and the spare keys are at home, call someone at home on their cell phone from your cell phone. Hold your cell phone about a foot from your car door and have the person at your home press the unlock button, holding it near the mobile phone on their end. Your car will unlock. Saves someone from having to drive your keys to you. Distance is no object.
Oh, and make sure you don't leave your cell phone inside the car next to the keys!