During one week multitouch interaction workshop with Nuno Correia at Aalto Media Lab I experimented with augmented reality posters.
I ended up using Unity 3D with Vuforia plugin and iOS. Using Vuforia was really straight forward and effective.
For casting shadows on a transparent plane I used ShadowDrawer shader by Keijiro Takahashi.
Source code can be found at https://github.com/ohss/AR_Poster.