Looks like Apple is trying to safeguard for webapps that may not be compatible with the taller format. So you'll see there's a black bar above and below the site when viewed from homescreen to keep the proportion of the old iPhone. Not a bad idea, but definitely not ideal for those of us who want to take over the whole screen!
As it outlines here, certain properties in the "viewport" meta tag will cause the not-fullscreen to happen. Mainly it's the "width=device-width" that keeps the viewport from properly formatting itself. You just need to remove that property from the "viewport" meta tag.
BUT this is something Sencha adds. So the only real way to remove it is to take it out of the sencha-touch-all.js file.
In the later versions of Sencha it looks like this:
Just run a search for "device-width" and you'll find it.
("viewport","width=device-width, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0, user-scalable=no");
Delete "width=device-width,"-- the rest can stay.
I normally don't like touching the Sencha file, but there doesn't seem to be any negative effects on the iPhone 4 screens.
("viewport","initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0, user-scalable=no");
Keep in mind, you'll have to re-save to the homescreen to see the change.
This temporary fix worked for me! Maybe down the line Apple/Sencha will adjust things...