Famitsu magazine’s report on a second analog pad attachment peripheral for the Nintendo 3DS is on the same page as their report of Monster Hunter 3G (more in this post), and it sounds like the primary function of the add-on will be to play that particular game.
In addition to adding a second analog Circle Pad to the device, the attachment will also add an R2 button. This is what it looks like:
So far, Monster Hunter 3G is the only game that looks like it will be making use of this add-on. Theoretically, other games could be programmed to use it, too, but as of now, details are scarce regarding who the peripheral is produced by and how wide an adoption rate amongst 3DS owners Nintendo have in mind for it.