It's a cradle that holds the 3DS and has both a second analog stick and an R2 button.
It adds about a centimeter to the left, an inch to the right, and an *additional* inch of thickness to the bottom of the unit, making the whole system almost 2 inches tall.
Here's a link to 01net's rumor that first spoke of this device and stated that it would later be integrated into a 3DS redesign/relaunch: http://www.neogaf.com/forum/showthread.php?t=441994
It looks like this:
This is where we are up to if you haven't been taking heed of 3DS news:
It roughly looks like this, *but notably thicker*:
Originally Posted by duckroll
Officially the cradle adds:
- A right analog slide-pad
- R1, R2, L2 shoulder buttons (the normal 3DS L1 button is not replaced)
- An earphone port at the bottom
Unannounced, but speculated based on CE Marks on the inside of the cradle:
- The cradle could be a battery pack or have a container for the battery
- The cradle also has wireless functions, possibly replacing those on the 3DS
New thread for new news. According to Adriasang, this week's Famitsu will detail the 3DS expansion previously reported by 01.net
Famitsu also has details on a new first party Nintendo peripheral that expands the 3DS with a second slide pad. This attaches next to the system's face buttons.
And the other rumors:
Originally Posted by The Other Rumors For Convenience