Randy Sarafan, who also happens to be the Technology Editor over on Instructables, gutted a remote-controlled car and swapped the parts for an Arduino board.

[Gilad] tipped us about his latest project, where he adds plenty of pneumatics and electronics into his wife’s car to remote control it. The brake/throttle pedals are actuated by pistons cont… ...

But if your car doesn't have a remote ignition, you can build your own remote start system that responds to a text message from an iPhone.