Of course the printed tags dont have to be 'physical' they can be embedded into Virtual Worlds such as Second Life, creating a really unqiue concept of importing/moving and visualisation objects in a virtual environment.
Take a look at the video below from cristiancontini for a really interesting proof of concept:
Augmented Mixed Reality: Second Life pops out from the Screen from cristiancontini on Vimeo.
See http://www.hitl.washington.edu/artoolkit/ for download and working examples.
See also our recent posts on Papervision Augmented Reality and Augmented Reality for Architecture using SketchUp.