Joel Webber:

"Clearly, you can’t ‘submit’ the entire page, because that would destroy your map and other context. Google’s solution is to submit a hidden IFrame, then gather the search results from it. Let’s say, for example, that you simply wanted to go to Atlanta. [...]
This HTML is loaded into the hidden IFrame which, when loaded, will punt a big chunk of XML back up to the outer frame’s _load() function. This is kind of a cool trick, because it saves the outer frame from having to determine when the IFrame is done loading.
I mentioned before that there was some advantage to be had by using a hidden IFrame over making direct XMLHttp requests. One of these is that the IFrame’s state affects the back button. So every time you do a search, it creates a new history entry. This creates an excellent user experience, because pressing the back button always takes you back to the last major action you performed (and the forward button works just as well).
[...]
I also think it bears noting that Google is pulling out all the stops to build rich web apps, no matter how weirdly they have to hack the browser to make them go. And I strongly believe that this is a trend that is here to stay — XHTML Strict/CSS/etc be damned. At the end of the day, what really matters to users is compelling apps that let them get their work done quickly."