When you slap a copy of Super Mario 3D Land into your 3DS it prompts you to perform a system update with firmware contained on the cart.
Among other less exciting things, this update added an option to the friends list menu to 'Join Game' - an option that will prove to be massively useful in future online-enabled 3DS games (which, ironically, Super Mario 3D Land is not).
As you might expect, it works rather like Xbox Live's 'Join Session in Progress' option, allowing you to transition straight from the system menu into a friend's online gaming activities.
The first game to make use of the option will be Mario Kart 7, out next month, and below is the first look at the 'Join Game' function in use.
It's not exactly explosions, aliens robots and zombies. But hey, if you're a 3DS with high hopes for its online future, this is kind of a big deal.