I'm not sure why people think that an app can replace hardware. Does your car have some sort of interface that allows for an app to unlock your car doors (this is what Viper SmartStart does)? If not you need to start there.
Similarly, apps can't:
1. Make your device replace infrared remotes to contol your audio video equipment (no IR emitter).
2. Enable your device to receive OTA TV signals (no tuner).
The list goes on and on. Just think for a moment and consider the chain of software and/or hardware needed to enable the functionality that you're asking about. What does your device include in its feature set? Does it have what it needs to accomplish what you're looking for? There are many apps that also require a service to enable their functionality (again, Viper SmartStart falls into this category as well as Schlage's solution and even Google Voice, among others).
Has anyone heard of or made an app to use your phone to remotely open the locks on your car like a key fob ?
DroneMobile allows you (through an app after install) to unlock, lock, and start your car. Don't think this is what you are looking for but think its along the same idea. Its not cheap but it works pretty good (when you have good service).
If you have questions please ask as I have one in my car and use it with my EVO.
I found several, the one that seems most promising does however cost $49.99 annually (Premium is more) but for a simple app that can unlock your can if you've accidentally locked your keys inside, is called Drone Mobile. Also found AutoNet Mobile, haven't tried it yet though (free app) and a very simple video on youtube which doesn't seem to require downloading or installing an app, according to the video, you lock your car with the remote, but keep holding the button down once the car is locked, at the same time press * 2x then # 2x then let got of the remote button. To unlock you then (close to your car window) press * 2x then # 2x then * again 1x, plan to try this one when I go out later