[...] Layar is a popular, freely available client which runs on both Android and iPhone. Along with providing free client they also have a augmented reality framework to publish layers to be viewed on the client. The framework provides simple extensibility (server-side) and web service integration point to developers who wish to publish their content to Layar. You can find more about Layar itself at http://www.layar.com.
There are many layers published on Layar to be viewed in the Reality mode where the 2D and 3D objects published by each layer is overlayed on the virtual mesh draped over the camera view of the phone. First it was interesting to look at all the layers published and then we started to experiment with this framework to expose ArcGIS data.
We have written some server-side code to enable the Layar application on the phone to consume ArcGIS data as both 2D and 3D objects and show it in a number of ways. [...]
Read more: http://blogs.esri.com/Dev/blogs/apl/archive/2010/04/15/Viewing-ArcGIS-data-in-Augmented-Reality-Application.aspx