An Augmented Reality client could run on wearable computers and smartphones/PDAs which now are getting close to the point where they have the necessary processing power, GPS receiver and cameras to function as Augmented Reality devices. Some interesting future hardware include cell phone laser projects and virtual retinal displays http://microvision.com
For 3D graphics A3DL would be used just like with Ter'Angreal, but for embedded devices like PDAs something more lightweight than crystal space would be needed to implement it. The Studierstube AR platform uses a reimplementation of Open Inventor for Windows CE.
Motion Tracking
Tracking the motion in the video from a camera is important to render 3d objects at the correct location. Tracking is processor intensive and often uses black and white markers to make it easier. An open source marker-based tracker used in AR is ARToolKitPlus
Ideally, markers would not be used and the client would download images from a site and compare them to the images from the camera to determine where to render things.
Location
Location could be stored as the available parameters of longitude, latitude, altitude and N' S' E' W' up/down position of camera.
The motion tracking module would keep track of the camera rotation in addition to moving objects, although this could possible be done by other hardware like GeoVector. If outdoors, location, time and shadows/light(intensity, color and direction) might possibly be used to track the camera in a N' S' E' W' up/down coordinate system. Location could be found localization hardware like GPS, or entered manually or implicit when entering a site related to one's current location.
When connecting to a site, physical location might translate to a location in the site's coordinate system.
Resources
Studierstube, has some open source software
Some cool AR videos