You can switch between search results, Gmail, Calendar, Reader and Google Docs by clicking on the big menu from the top of the page. Google uses AJAX to make the switch almost instantly, as if you were using a desktop application.
"Our guiding principles were fast and fluid. We think we've achieved both, thanks to some AJAX magic made possible by the iPhone's Safari browser," tells us Google Mobile blog, where you can find screenshots from an iPhone.
If you have an iPhone, you can see the new interface by going to google.com. Otherwise, iPhone's user-agent is the only helpful alternative. [Update. Garett Rogers found an URL for the new interface: http://www.google.com/m/gp, but it doesn't work well in any important browser.]
