Readline also has a vi mode, which I've heard is pretty good.
I agree that a good OS should allow you to choose your editing keys (either vim or emacs style) and not allow applications to step on them. Unfortunately that's a feature that most users probably wouldn't care about.
On the other hand, the feature wouldn't hurt them; if a default user-friendly interface personality is provided with the existing behavior, they don't even have to know that the the system is swappable.
If my hands weren't so full and I had any special level of expertise with GUI development, this is an itch I would find worth scratching - I don't imagine this could occur anywhere but linux and bsd, though.
I agree that a good OS should allow you to choose your editing keys (either vim or emacs style) and not allow applications to step on them. Unfortunately that's a feature that most users probably wouldn't care about.