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Here are a few options you can use for proximity + Push button start and of course you can always replace the ignition key switch with a number of buttons or switches and just use a button for the start.
http://www.digitalguarddawg.com/
http://www.kptechnologies.com/products.php#/18
http://www.auto-hub.com/stardrive_push_start_ignition.html
question is how do you get around the steering wheel lock and how to you access the accessory option like if you just want to listen to the radio. cool idea indeed though.
If you look at those links there is key that you get cut that stays in the ignition to keep the steering wheel unlocked but the module keeps the car off until you press the button once. Then you are in ACC mode, when you press i again it starts. When you are getting out of the car you press it once and the engine stops and you are in ACC mode again with the radio playing etc. When you open the door it cuts off completely... pretty slick.