http://www.ign.com/articles/2013/11/13/3ds-update-brings-miiverse-unified-accounts

Today's Nintendo Direct revealed details about a new 3DS update that will bring both Miiverse and a unified Nintendo Network ID to the system.

Register the same Nintendo Network ID on both your 3DS and your Wii U, and you can combine your balance on both systems, and buy games from the eShop on either. Previously, each system used a different wallet. There will also be a measure in place that allows players to access and use the remaining balance on either piece of hardware.

Miiverse will also be coming to 3DS after this update - it looks like 3DS games will be getting their own Communities, and after registering your Nintendo Network ID on the 3DS your activity will transfer across.

Nintendo promised that eventually more Nintendo services will use your Nintendo Network ID - this, it seems, is a first step to greater unification across 3DS and Wii U. Nintendo didn't say it would be coming, but we can assume it'll be soon.

Can't wait for this to happen. I got more money on my 3DS then my Wii U