Just recently I stumbled over some exciting videos showing how augmented reality on mobile phones can be used to overlay the real-world with additional information when seeing it through the mobile:
This looks fascinating on the one hand as it finally runs on mobile phones today, but the idea itself is also kind of old on the other hand:
I can imagine that applying AR may make sense in specific situations but how do indicate to you users that there is virtual information "behind" the current real-world view? How frustrating would it be running through a city watching it entirely through your mobile phone without finding any augmented information?
In that sense I still find the approach of tagging places with barcodes or NFC more convincing, e.g. as ServTag or several NFC projects show.