Not long ago my brother wanted to know if I could make a van alarm that would send a text message to a cellphone notifying that the car had been broken into.
Are there '2g/3g cellphone modems' that you insert a sim into and connect to an arduino's serial pins? It would have to be able to:
Well, that's a lot for the cellphone modem to do. The cellphone modem and other stuff on an Arduino would make the Arduino able to do all that stuff, easily.
I'd argue, though, that the activation of the system should not require sending it a text message. Activation should be automatic when the ignition is shut off.
Finding the vehicle after it has been stolen, when you forgot to arm the system is much more difficult than finding it when you did arm the system.