The creators of the great Reactable multi touch music interface released a mobile version of their software for IPhone/IPod and IPad.
The original Reactable hardware is running software sided with reacTIVision an “an open source, cross-platform computer vision framework for the fast and robust tracking of fiducial markers attached onto physical objects, as well as for multi-touch finger tracking. It was mainly designed as a toolkit for the rapid development of table-based tangible user interfaces (TUI) and multi-touch interactive surfaces.”
More info here: http://reactivision.sourceforge.net/#usage
reacTIVision itself makes use of the TUIO protocol an open framework that defines a common protocol and API for tangible multitouch surfaces: http://www.tuio.org/