They're called AppMates, and so far these Cars 2 toys seem to be the only ones that exist. The idea is simple - you have a physical car toy that the iPad can sense. You download the free racing app, and you can race your real car toy on the virtual race track on your iPad.
Like I said, not that awesome right now. But think of the possibilities! You could theoretically build a manipulative that was not only sensed by the iPad, but also could detect what was displayed beneath it, giving you two-way communication of a sort. I once had an idea for a book that would act as a companion to existing popular Nintendo DS games, outlining activities that combined the game with the real world to simulate scientific activities - for example, using Viva Piñata as a location for doing observations similar to those a biologist would do in the field. That sort of idea plus this technology could do some really amazing things!
This post cross-posted to The Cognitive Informalist.