The 3DS add-on that fits a second analog stick to the device is called the Circle Pad Pro, according to Capcom's Tsukasa Takenaka. This confirms speculation from an earlier report that caught the trademark filing and drew the obvious conclusion. In Japan it will be called the "slide pad expansion."
Speaking to Eurogamer, the Resident Evil: Revelations assistant producer said it was a "no brainer" to support the device once they learned about it. "When we decided that we thought, we have to do it fully, we have to create a whole other control scheme based just around this peripheral." The Circle Pad Pro layout is Type D, and you can select an option to calibrate the add-on.
Meanwhile, Joystiq reports that Capcom isn't planning a Circle Pad bundle for Revelations, so you'll need to buy it separately. And in case you were hesitant after the RE Mercenaries 3D kerfuffle, yes, you can delete your save data with ease.
Takenaka calls the Type D with Circle Pad "the right way to play" Revelations, which seems reasonable enough. On the other hand, the existence of the peripheral means a split userbase and the looming shadow of a possible hardware revision for the 3DS device. Given Nintendo's recent financial troubles due partly to poor 3DS reception, it's hard to tell if a move like this help or hurt the device.