Dubbed the ‘Expansion Slide Pad,’ the add-on is manufactured by Nintendo and cradles the 3DS to add dual analog support and two shoulder buttons. Early reports suggest it will also sport an earphone port and a new battery. The pad has been developed specifically for Monster Hunter to give players better control, but it could also feature in future games too if other developers decide to use it.
Nintendo has obviously been keeping an eye on Sony and its new hand-held, PlayStation Vita, which benefits immensely from the implementation of two analog sticks to deliver an intuitive, console-like gaming experience.
Source: http://www.andriasang.com/e/blog/2011/09/07/3ds_slide_pad/