Update: Goggles no longer exists. See How do I read QR Codes? for alternatives.
QR Codes should have been implemented this way in all phones, but sadly isn’t.
If you have an Android phone you can install Goggles (if it’s not there already) and tie it to the camera app, to get a similar experience. You can of course read QR Codes within Goggles as well, with a quicker response (no pun intended).