Kottke points out the benefits and convenience of one-handed iPhone use.
Most of the time I’m using my iPhone, it’s one-handed. Left, specifically, as it comes from the left pocket. Instapaper was designed for all-left-handed operation because that’s how I use my phone most of the time: left fingers around the back to hold the right edge, left pinky holding up the bottom edge, and left thumb to hit buttons or rest on the top-left corner when not needed (usually when using tilt-scrolling) for extra grip.
(For what it’s worth, I’m right-handed.)