I thought I'd post some photos to show what I'm talking about 
iPhone in palm for one handed use
Left thumb bent for comfortable use of lower left of screen
Left thumb extended for comfortable use of top right of screen
Left thumb stretched as far as it will go (not comfortable!)
This is from a while ago, but left thumb extended as far as possible across a Galaxy Nexus
iPhone in palm for one handed use

Left thumb bent for comfortable use of lower left of screen

Left thumb extended for comfortable use of top right of screen

Left thumb stretched as far as it will go (not comfortable!)

This is from a while ago, but left thumb extended as far as possible across a Galaxy Nexus
