[gumby]

The beginning of the chain is a human -- the user! And at the end of the day this is all about the user!

Once you know what the user wants it's "easy" (well, somewhat easy anyway :-). It's the iterative process of understanding the user in an unconstrained opaque ad hoc UI that is big-H Hard.

The problem with these spoken interfaces is not just that we don't know what we don't know, we don't even know what we want to accomplish!