The concept of the fold goes back several mass mediums to the days of the newspaper. However, there’s no one true fold in the mobile web. Obviously the inverted pyramid still applies and the most important content should be displayed first, but whether or not that content is visible on page load doesn’t matter as much as it might in the desktop web.
First, there’s too many device heights on the market to establish a rule-of-thumb fold (think of Blackberry’s stubby screens and a long screen like the EVO4 or DroidX). The iPhone browser has a screen height of 416px but that’s just one device of many similar but not exactly the same screen heights.
Secondly, the first thing a mobile web user is prone to do is scroll vertically. Whether the user flicks their touchscreen, scrolls with their Blackberry Pearl, or incrementally chugs down the screen with a D-pad, the fact remains mobile web users understand there’s more content to be had.
It’s ok to think about some invisible fold, but don’t get too caught up if everything can’t fit above it in your design comps.