With the new cars coming out, where you don't actually put a key in an ignition (I've seen new Nissans with this feature, I'm sure there are more). That also unlock your vehicle via a close proximity (allthough I'm not sure what that proximity is... ) ,and if you do happen to have some incident where your magic "key" gets locked inside, there is the code pad on the driver door as well. This really isn't a new tech, its just finally being adapted for use in cars finally.... So then the question isn't if an app can be developed, because its not so much a software thing, as it is a hardware. I agree with the previous statements about some setup bluetooth or wifi.... allthough this really would only be practical at home... since your not likely to have Wifi when you goto WalMart lol. Also not sure that I'd want my car controls to be accessable via the internet for a hacker/car thief combo. It would be very convienient if the phone had an rf transmitter and an app that removed the need for an addition to my key ring.
Keyless remotes have been around for a while on higher end cars. They are now available on the fully featured models of many new cars. I have it on my Mazda 6 and love it. The keys stay in my pocket snow, rain, or shine. The car won't let you lock your keys in the car.
I think the only way to get an app on the phone to do something like this would be if the car was connected via an *n star type system. The commands would be sent via the internet to the car.