Beginning this year I published an iPad 3D model viewer (works on iPhone as well, but there is less real estate).
3D Model View is an ios 3D model viewer that supports several 3d formats and allows realtime material editing amongst other features.
The next update I'm working on will include an AR feature, something i'm thrilled about!

Any 3D object just 'sitting' on a marker, (in this case my phone).

And what do you know: It is the Doc's Delorean from Back to the Future, how cool is that!

In fact it could be any object in a format the App can load and handle..

B.T.W, sorry: no audio

The rendering is done in OpenGL, using custom shaders. 
The camera image is processed with Pattern recognition techniques to determine the virtual objects's position in the real world.

More info about the version that is currently released (without AR) at http://3dmodelview.com