Despite Nintendo downplaying the Circle Pad expansion at its press conference, details of its existence emerged shortly after the media briefing with support from third-party developers.
In addition to Monster Hunter 3G, which was first announced alongside the Circle Pad expansion in Famitsu, other games that will support the new peripheral include: Resident Evil Revelations, Metal Gear Solid: Snake Eater 3D, Ace Combat 3D, Dynasty Warriors Vs., and Kingdom Hearts: Dream Drop Distance.
It sounds like you don’t need the add-on for playing those games but it will surely “enhance” your 3DS experience. The device will retail for 1,200 yen ($20) in Japan and requires a AAA battery for it to work. No word on a North American release date and pricing.