Well, I'm pretty sure I remember that in the press conference they mentioned that the steering wheel would be packed in with Mario Kart, and the Zapper would be packed in with Resident Evil:UC. I'm pretty sure they might release other titles along with the attachments.

And of course, like so many other people said, the attachments are only casing for the controllers in an attempt to put you just a bit more into the game experience. They are otherwise completely unneccesary. Considering it's all optional, there really isn't anything to be complaining about.