Update: Nintendo responded to our inquiry and reminded us that it does not "comment on rumour and speculation.", so we'll leave you to make your own decision about this one.
You may want to have your salt shaker handy for this one.
A French fansite is reporting that Nintendo might be working on a cartridge that will allow a 3DS to be used as a secondary controller for the Wii U console.
Nintendo Master claims that an unused cartridge patent for the DS (reported by Siliconera back in 2010) could possibly be used in conjuncture with a Circle Pad Pro to turn a 3DS into a secondary Game Pad controller for the Wii U.
There are a couple of problems with the rumour as it stands, however, namely that the cartridge pictured in the patent sticks out a fair bit from the DS model and so it could not fit onto a 3DS with a Circle Pad Pro attached to it.
...also, it's shown plugged into a DS Lite and not a 3DS.
The minor quibble of logistics aside, there is some credence to the thought that Nintendo might be looking to further integrate the 3DS and Wii U. The upcoming Monster Hunter 3 Ultimate (due out in the US and Europe in March) will support a cross play between the two systems, so allowing a 3DS and Wii U to play together isn't so far of a stretch.
Nintendo has yet to comment on the rumoured cartridge, so we've reached out to see if it could shed some light on whether this is future functionality or simply wishful thinking.