UPDATE (8.21.08): It turns out we got some wires crossed on this story: According to a Red Octane rep., there is no new peripheral. There is an option in the Studio Mode to add keyboard sounds but that comes from the guitar. The input port on the drums is for any other instrument you would like to add to the tracks. Our apologies for misinterpreting.
We just got the hot word from Neversoft's Brian Bright, and we have some big Guitar Hero World Tour news: A new peripheral is coming. The new periph. is specifically for the game's Studio mode, to help you make music, and while Bright wouldn't go into detail about what it will be, we are speculating that it will be a keyboard. Again, not for use in the game, but for use in the game's studio. We think it'll be a keyboard because, simply, that's the best way to control music input... but, hey, we might end up being wrong. Look for an official announcement in the upcoming months. We'll let you know more when we know more.