Last week Google updated Google Earth to include the Sky above you and it looks like they also included a unpublished Flight Simulator, you can take off and fly around the world. You need version # 4.2.0180 you can get it here http://earth.google.com/sky/index.html
It work on Windows Mac and Linux. To get the Flight Sim working for the first time Open Google Earth the press Ctrl+Alt+A for windows for Mac Command+Option+A
After you run the Flight Simulator for the first time you can restart it by starting Google Earth then go to Tools > Enter Flight Simulator
Here are the keyboard shortcuts
And another screen shot.
Update 9-3-07 Here’s a video from the Google Earth Blog
Update 9-1-07 Here’s a video of it in action by Eric S. over at http://www.insidedesign.info/blog/