Nintendo 3DS owners can finally share their eShop account balance with their Wii U, as the promised system update adding Nintendo Network ID support has arrived today. The update also brings the Miiverse social network to 3DS, to chat about games and share pictures.
Update 7.0.0-13U is available now, and should download automatically when you have a wireless connection and have SpotPass enabled. Otherwise, you'll need to can update it manually.
You'll need to create a Nintendo Network ID on your 3DS, and it'll only share the eShop balance if it's identical to the one on your Wii U, so do be careful. Yes, Nintendo's wonky way of linking accounts to hardware is still making things more difficult than they need be.
Creating a Nintendo Network ID isn't mandatory, but people won't be able to download free software like apps and demos without one.