So this happened to me last night - the
3DS Black Screen of Death. It pops up and you have to power off your 3DS. I was playing
Ghost Recon Shadow Wars when it happened (twice). It may have something to do with brightness and power save, as I had turned up the brightness to 5 (from 3, my usual setting) and activated the power save option. When I went back to my original settings, the game ran fine.
Tried out the StreetPass function yesterday and managed to score four Mii's inside an EB Games, perhaps not surprisingly. Got another three at lunch time today - all of whom where female (possibly not IRL). TIP: Before you start cruising with StreetPass, you need to play the Puzzle and Quest games at least once. The new Mii's won't show up in those games otherwise.
Note the grey-haired Mii second from the right. I smiled when he introduced himself as our ex-Prime Minister.