Not only numpty physics, but things like sketching or editing graphics would be difficult to use without a stylus. I haven't seen an international art movement devoted to fingerpainting yet. Also, just for editing, let's say you want to move a few words. The only decent replacement for a stylus would be a mouse. Sure, there are keyboard commands for that, but not everyone wants to learn vi.

Hey, how about a cursor directed by brainwaves! That's reasonable for the N900, isn't it?