My ideal arming and disarming of the car alarm is when I walk into range of it. I don't want to have to press a button. I've been hoping I could use bluetooth. If my phone is connected by bluetooth it disarms the alarm. If my phone is not connected by bluetooth it arms the alarm(Because you may want to turn off your phone while driving I may make it detect if the car is running so the alarm will stay disabled if you turn off your phone. If you start the car with the alarm armed, it would not disarm.)
I have a iPhone, and most of the arduino Bluetooth adapters I've found wont connect to it, and I would also like to have the arduino serve a web page with arm/disarm controls and the ability to view a log of alarm events.
I just found the wifly wifi shield that can be setup to be a AdHoc network. In this mode I would check the devices the arduino is connected to against a access list. If the device is in the access list it would disarm the alarm.