Updates include an emails status (sent? not sent?), connection status and offline/online.
The bar fires up when you fire up the browser, then disappears after delivering the goods.
The goal, according to thinkmobile.com, is to have native software feel more like native applications, greasing the wheel for integration of Chrome.
It's an opt-in feature, too:
You can see this improvement by visiting gmail.com from your iPhone or iPod Touch’s browser (iOS4 only). Don’t see the new changes yet? Try clearing your cache and refreshing the page. And if you like using Gmail in mobile Safari, get to it faster by tapping the “+” at the bottom of the screen and then “Add to Home Screen.”