Yesterday, we found out that the Japanese version of Super Mario 3D Landpacked a mandatory 3DS firmware update, which enhanced security (of course) but also streamlined the friends list, adding a new "Join Game" option.
Today, upon the arrival of our US review copy, we discovered that the same firmware update is loaded on that version, as well, and offers the same amenities. You can see the "Join Game" button at the top of the screenshot above. It's faded out because the game I had running at the time doesn't have multiplayer.
There's more 3DS updating to come, as a more comprehensive update is due this month to add video recording features, along with updated StreetPass games, DLC functionality, and other eShop improvements.