I vaguely recall someone posting examples of 'open nav' icons (eg Path and Facebook) showing an emerging de facto standard. Link?
"Unless our navigation’s arranged in a grid (and so we should use a grid icon), I’m putting my weight behind three lines because I think it’s most recognisable as navigation to the average person."
The three-lines icon is certainly very popular, as can be seen in this collection of mobile navigation icons I gathered together on Dribbble.