The mobile marker based applications are very rare but could end up being a very cool addition to existing applications. Some of us already now the 2D QR-codes. The market is very open and up for graps.
Check this site out:
http://studierstube.icg.tu-graz.ac.at/handheld_ar/
The Windows Mobile variant
http://studierstube.icg.tu-graz.ac.at/handheld_ar/artoolkitplus.php
