One system that some drawing programs on the iPad use is to completely hide the UI most of the time. Gestures and hot spots cause the UI to manifest itself when

One system that some drawing programs on the iPad use is to completely hide the UI most of the time. Gestures and hot spots cause the UI to manifest itself when needed; most of the time the screen is empty except for the painting. I can imagine a word processor that works like this. (Code editors often do act like this, but use command keys in place of a GUI.) Discoverability is the big problem here, at least for casual users.