I have just found out about Google Earth's "G-Force" navigation mode. Just hit Control+G (Command+G on a Mac) click and you can "fly" using your mouse as a joystick (first you must gain some speed by right-clicking for throttle control). Turn on terrain elevation, turn off all other layers and you'll have a great flying experience.

Command+T to go back to regular navigation mode (even if I don't see why you should ;-).