A recent news post on Travelgumbo mentioned Flight Map, an Android app that can give you a live flight map display on your phone, even in "airplane mode," with the cellular radio features turned off. It works by combining known information on thousands of airline flights and readings from the GPS system.
I tried it last night on a flight from New York to Madrid, keeping it plugged in and with the airline's display on the video screen. Despite a few glitches, it works pretty well. It consistently matched the distances from the start point and to the endpoint as shown on the monitor, and after a serious lag at the beginning of the flight, it pretty well matched the altitude and speed...and it's hard to say which was more accurate. For instance, Flight Map gave our altitude a 33,950 feet and the plane said 35,000. Could be the plane software is rounding off, could be the GPS.
I think most of the big differences (showed us at 4500 feet for most of the first hour) was probably due to lags or difficulty in reading the GPS satellites.
The other issue with the app, which I hope will be corrected in future editions, is a very unintuitive interface, with practically no instructions. Not all the information you expect from a flight tracker was there, and the info was grouped in confusing ways.
But hey, for 99c, especially if the flight doesn't offer its own real-time tracker...