I've done a bit of work with AR in conjunction with flash and papervision. I run a rental DVD delivery service, and the eventual aim is to be able to have a proper look at the DVDs as you would in an actual shop.

Check it out here:
http://www.youtube.com/watch?v=Sg2USM0MTC4

And have a go here if you like:
http://www.videoemporiumdelivery.co.uk/virtualdvd.html

Couldn't get it to work using the browser on the n900 though, how do you get flash to use the camera as a webcam?