In December, Nintendo will bring their Nintendo Network account system to the Nintendo 3DS, allowing 3DS owners to either create a Nintendo Network ID for their 3DS, or to use their existing Nintendo Network ID from Wii U on the device.
Using the same Nintendo Network ID across both consoles will allow you to share your Nintendo eShop credit between the two. Additionally, Nintendo mention that, in the future, you will require a Nintendo Network ID to download free demos and other content on 3DS, as well as post to Miiverse, which is coming to 3DS soon.
However, there’s a catch—you’ll only be able to create one single Nintendo Network ID account per 3DS console. Nintendo say that, if you’d like to transfer your account to a new Nintendo 3DS system, you’ll need to use the “Transfer title and data” procedure.