Many people want to use iPhone's camera directly, instead of its ImagePickerController. So, people have used PLCameraController class, which is one of private framework classes. After the iPhone OS is updated to 3.0, however, PLCameraController class is modified and the old method to get preview does not work anymore.
There is a nice thread that discusses how to use PLCameraController on iPhone OS 3.0. See it here.
I succeeded to display the camera preview on my iPhone by following the thread. The code works well on both iPhone 3G and iPhone 3GS. On 3GS, the focus rectangle is automatically displayed as shown in the capture image.
However, the raw data, which all the AR developers may be more interested in rather than the preview, is not accessible until now.