There's a system like this out at the moment.
And it's also an immobiliser/alarm.
You can SMS it to immobilise or lock the car.
Get position via SMS of the car, or jump on the net, and view a map of where the car is.
The downside, the system is around $1000 (But, it's also an integrated alarm/immobiliser) and you can remotely check status of the car, and arm/disarm features.
So full alarm+immobiliser, with tracking...
Can't remember name atm... :S
Also, pre paid isn't recommend by alot of people, as you need to keep getting access to the phone to recharge it (Or some you can call up and do it with an operator) but again, plan is easier, as you know it won't run out of credit.