The fact of the matter is that your eyes look directly in front of you and your fingers are typing at 90 degrees to that.

I don't know from experience but I can only guess how uncomfortable it would be to have to lift my arm up to have manipulate on screen objects as opposed to using a mouse and/or keyboard. Maybe I'm just lazy..

I don't think the iPad will displace this traditional method of input unfortunately. The mouse and keyboard will be around for a long time to come.