Mum was the word from Nintendo on the growing nightmare known as Nubageddon at their 3DS press event today. But those observant folks over at Andriasang noted an update to Big N's hardware site containing juicy info on the peripheral.
The "Slide Pad" will require a single "AAA" battery to function and will only work with compatible games. Titles currently confirmed to be compatible with the Slide Pad include Monster Hunter 3G, Resident Evil Revelations, Metal Gear Solid Snake Eater 3D, Ace Combat 3D Cross Rumble, Dynasty Warriors VS and Kingdom Hearts 3D. The peripheral is listed at a price of 1,500¥ (roughly $20 US), which sounds pretty reasonable considering I don't expect more than two dozen games will use it without a hardware revision to the 3DS to force the issue.
3DS Slide Pad Attachment Formally Announced [Andriasang]