I'm not sure an arrow would be best in this case. In the browser app it brings up an actual mouse pointer, which is something we won't get. Basically the button would turn on/off the kinetic scrolling so that the click and drag action would not scroll, but instead select text.

So I could imaging an icon that stands for: "Turning scrolling on/off" or "Turning selection on/off". But I'm really not sure how it should look like. A pointer? A finger? An "arrow cross"? Or maybe like this selection icon in acrobat reader?

What do you think?