There is a very simple Jailbreak app called NoAccSplash that prevents the annoying Accessory Connected screen when you plug your iPhone into a dock or via USB into a car. There is no UI or configuration required.
The standard external iPod controls are really slow and inefficient. You can't search for music and the touchscreen controls on my car stereo are really, really, painfully slow.
Now I can easily plug my phone in (& before taking the car out of park) quickly browse to the song or album that I want.
The free app is by Brandon & can be found in the BigBoss's repo.