Marco Gallotta covers the details on his blog. It's fairly rudimentary (check out the list of controls here) but it lets you fly through all of that gorgeous Google Earth 3D imagery. Try turning on the 3D buildings and terrain and flying around a major metropolitan area.
We had trouble getting it going until we followed a commenter's note that said to head to San Francisco International Airport (SFO) and try activating the controls from there. Worked like a charm, and we were crashing into the ground in no time. Soon we'll all be playing Google Earth, The MMO. Video of the Google Flight Sim after the break