I think Apple may allow kayboards and mice but not right away. They might very well wait two or three years to first "educate" everyone about how tablets are supposed to work.
Just an example: I don't think it would harm the iPhone now if Apple started to sell pens. It would have hurt the iPhone when it started.
On the contrary, I think the longer the platform goes without kb/m, the less relevant they become. At this point, with so many people so used to touch* and typing emails on their iphones about as quickly as they did on their blackberry, how many people in Apple's market would choose the hassle of a BT chiclet keyboard?
(*the most common usability complaint I hear RE: Android, is apps that don't expose functionality through on-screen UI, iphone-style, but via the 'menu' button)
As for pens, Apple may be content to let third parties handle that. User expectations don't get confused, but much of their power can still be harnessed.
Just an example: I don't think it would harm the iPhone now if Apple started to sell pens. It would have hurt the iPhone when it started.