You have got to be joking, right? There is no way on earth that tying onto a multi touch screen would be as fast or natural as typing on a conventional keyboard. And it's not just because it's a new concept and we're not used to it. Try typing with your hands at near vertical and see how easy it is. Yes, if you're the kind of person who wants to type one-fingered monosyllabic replies on a chat site, maybe it would be fun, but otherwise it's a no starter. (Think about how you'd insert the cursor into a word on screen if you wanted to correct the spelling - your finger would obscure the point at which you wanted to do the insert. That is just one of a thousand common actions that would have be solved first before a real switch from mouse to multi-touch screen functionality could be effected.)
You also completely underestimate what is involved in porting a sytem interface which has been entirely mouse and kepboard operated since its inception, into an interface which is fully-functional through touch (there seems to be an oversimplification in general here, between the multi-touch capabilities offered by a track-pad and the notion of a touch screen that would fulfill all the functions previously carried out by mouse/keyboard - the two things are very different).