Frequently enough, something gets erased because I rest my hand on the screen (while using a stylo to draw on the screen). When I discover that something was erased like this, it's too late to "undo" and bring it back, because I'd also undo all my later work. An option for audible feedback (make it fun, like the sound of a sponge, whatever) would immediately let me know that stuff get erased without my intention.

PS: kudos for letting me erase via my finger though, excellent usability touch, sooo much better than having to select the eraser tool