The airport view doesn't show the aircraft at elevation. But if you Google a flight number (say, DL 161), before the first search result you'll find links to "track this flight via Yahoo, MSN, or fboweb." At the moment, I don't actually recall if it's actually Yahoo and MSN, or some other sites, but you want to click on fboweb to track the flight. At the fboweb tracking page, click on the link to the right, below the small map, to track the flight via Google Earth. The aircraft icon will update its position every 60 seconds or so, and will leave behind a trail indicating where it's been. And when you pan the camera down to see the landscape relief, the plane is at the proper elevation. You can watch the elevation decrease as the plane approaches its landing.

This only works for flights in progress. If you try to Google a flight number for a flight that is not in the air, Google won't recognize it as a flight number. Also, as far as I can tell, there's no way to track a flight like this by starting within Google Earth.