I never had this problem with the iPhone 3G, and neither when I first got the iPhone 4, but now when I plug in my iPhone 4 to my car radio via USB, it skips and pauses. It doesn't show on the screen that the pause icon was taped or anything, it just pauses. Also, this only happens if I play music from my library or Pandora, never when I play music from YouTube or any other app.