Extremely well thought out project, nice job. Although it might not be the safest idea, being able to send an SMD message to the phone that can completely disable the ignition would be nice.
There's a commercial system here in the US called Onstar that uses cell phone service to connect to a help center which monitors the car in case of an accident or emergency, if you have the proper authorization codes you can even have them unlock the doors for your with a phone call if you lock your keys in, things like that. A new feature on some vehicles for anti theft is Onstar can even disable the car if it's been stolen, even if it's being driven.