Both Ford and GM have recently announced SDKs for integration with their cars.
The Ford developer site contains what looks like a well thought out suite of development tools, with an AppLink emulator and series of SDKs for various platforms to connect to it. There is also a suggestion that the OpenXC platform is supported.
Meanwhile, the GM developer site seems somewhat broken and uncared for, so I cannot get any real information from it - the registration doesn't work in my Firefox and their blog has run out of momentum.