Greg Duncan in the Channel9 MSDN website write about the GART – The Geo AR (Augmented Reality) Toolkit for Windows Phone 7.1 ( Mango ) .
“GART was created to help people quickly and easily build Augmented Reality applications for Windows Phone.
This kit is different from other AR kits in that it enables what we call “Geo AR”. Where other toolkits place virtual things on top of specially printed tags, this toolkit places information on top of real places in the world around you. It does this by tracking where you are and the direction you’re facing.”
Greg has also embedded few videos to show the demo of the App using GART .
Read more about The Geo AR (Augmented Reality) Toolkit for Windows Phone 7.1 from Channel 9 site