Just a heads up for those of us in the UK. It seems that the Argos Streetpass relays have stopped working automatically, and you now have to register your phone with O2 wifi in order to get that green light.
This, of course, dramatically reduces the likelihood of getting a Streetpass outside Argos, as you have to be pretty committed in order to sign up.
Compare: Walking past Argos with a 3DS, getting a green light.
To: Walking past Argos with a 3DS, hanging around to register your phone with O2 Wifi, waiting for a validation code, getting a green light.
This annoys me as I was getting quite a few hits there.
And of course, when I finally got that green light, it was myself (2nd 3DS) from a few weeks ago. (Warrior's Way army size was 300k, compared to the 1m it is now.)