Read Write Web describes Layar as:
the jaw-dropping Dutch Augmented Reality browser... Of all the Augmented Reality apps we've seen so far, though, Layar is the most exciting because it's a platform.
I am inclined to agree and have applied for a developer key for their sdk. I think Filippo might work well using Layar. So far, my attempts at implementing Filippo for the Google Developer Challenge 2 (the due date is this weekend) have been frustrated :-(
Stay tuned to find out how I get on.