Are there any good tutorials or sample applications out there that demonstrate how to make an augmented reality iPhone application?
I doubt exactly such a thing exists, but what you need to do is look at the location and camera frameworks for the iPhone, and go from there.
Basically, you will create a UIImagePickerController (the Camera class) and overlay information on the view, via a custom .cameraOverlayView (which is a property of UIImagePickerController in 3.0).
You will probably want to get information (location, heading, distance, etc.) from the GPS of the device, using the CoreLocation framework.
You will also probably want to do some web server interactions, and for that I suggest ASIHTTPRequest.
Those are the basic tools needed to build an AR application.