[penartur]

I believe i have a better idea.
2 icons; one for pencil/eraser, other for zoom/pan. So when you will tap on the pencil icon once, the pencil mode will be activated; when you will tap on the pencil icon twice, the eraser mode will be activated; when you will tap somewhere on the free space on the statusbar, "nothing" mode will be activated.
So there will be no need to redraw (as opposed to popup menu solution); and for two of five actions there will be one tap needed instead of two (also only tap instead of two for e.g. entering the eraser mode if the pencil mode is used); and it is imho simplier to understand. Cons are that there will be two icons instead of only one.