Marco Gallotta, who found an an easter egg in Google Earth: "Moving on though, you can get a quick start by holding Page Up for a few seconds to increase to maximum thrust (thrust meter is the left bar of the lower-left meters). Once you've accelerated to a sufficient velocity use the arrow keys to take-off. The keys are in reverse as one would expect with any flight simulator, so use the down arrow to take-off. When you've gained enough altitude then stabalise the aircraft to a straight flight path. It can be rather tricky to get the hang of as the controls are quite sensitive."
How to see this feature. Make sure you have Google Earth 4.2. Open the application, click on the globe and then press Ctrl+Alt+A. You should see this dialog that lets you choose one of the two aircrafts (F16 "Viper" and SR22) and an airport.
To fly, you need to read this list of keyboard shortcuts, but you can also use a mouse or a joystick. "To disable or enable mouse controls, left click (single click on a Mac). Once mouse controls are active, the pointer shape changes to a cross on your screen."