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.
Here's the initial view from London Heathrow Airport:
... and here's a nice view from Kathmandu:
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."
Marco Gallotta, who found this feature, has some tricks: "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."
This easter egg could become a standard feature in the next versions of Google Earth and it will bring even more fun to the application.