I have a question I haven't seen answered anywhere: How does these glasses work with focus? When I'm looking (and focusing) 5 meters away, how can I see an object overlaid there? Surely, I'd have to focus on the glasses, 3 cm from my eye, no?
You can use lenses or mirrors to change the apparent focal distance of an object. Google's implementation is rumored to use holographic optical elements instead, probably for compactness: http://www.google.com/patents/US3940204
Iris dilation is primarily a response to light levels, not focus depth. Focus comes from stretching the lens behind the iris, which is possible to sense externally, but much more difficult.
I'm not really sure it's active, but still, the question of how you get something on a surface to become visible when you focus meters beyond that surface remains.