It has been revealed that Nintendo has plans to release this!?... thing:

This is called the 3DS Expansion Slide pad.  The first game to utilize this fugly peripheral will be Monster Hunter(tri) 3G.  This is a ground up remake of Monster Hunter Tri released on Wii featuring new monsters, levels, and weapons. Go to Kotaku for more info

This cradle like add-on will add not only a right analog stick, "slide pad", but also an R1, R2, and L2 buttons.  The 3DS' original L1 button will still be reachable with the cradle attached.

Since it is a cradle, it does surround certain parts of the 3DS. It appears that the game cartridge slot is blocked by the 3DS Expansion Slide Pad, meaning that you must take the 3DS out of the Expansion Slide Pad to change games. Apparently, the wireless switch is also blocked.

Nintendo is expected to officially announce this at next week's Tokyo Game Show on Tuesday the 13th where Nintendo is holding a pre-show press conference.  This show will be streamed on and will probably be shown on the Wii's Nintendo Channel.