Nintendo promised a features-adding 3DS firmware update before the end of the year, and a features-adding 3DS firmware update is exactly what we now have. The system update is available for download now, bringing 3D video recording (10 minutes worth), new StreetPass puzzles and a new dungeon for the StreetPass RPG-lite.
The big surprise is the addition of 3DS Accomplishments, which appear to be the Nintendo take on Achievements/Trophies. Right now these appear to be restricted to Mii Plaza milestones, such as meeting Miis from 10 different countries, but it seems like a safe bet that we'll see them popping up in games before long, possibly even Wii U games further down the road.
Other new features include DLC support, 3DS demo downloads, Sleep mode automated downloads and a tool for transferring content from one 3DS to another. You'll find the System Update option tucked away in one of the handheld's System Settings sub-menus.