Ford has just released OpenXC - an open-source software toolkit that will let the hacker community play around with the computer systems that run your car. While it may be awhile before useful apps appear, the open nature of the system should lead to custom apps that give drivers far more control over their car's performance.
K. Venkatesh Prasad, of Ford Research and Innovation, explains that most drivers use only speed, gas level and engine status. With the open source, many more apps will be developed and be able to control the auto in every aspect.
From a security standpoint, it is unlikely that Ford will approve of custom apps. They would be unsafe and hackers would easily by pass security much as they do with cell phones.
In theory, OpenXC should work with any make of car. Several auto companies have expressed expressed an interest.