Google Earth Flight Sim
Use Google Earth data, including but not limited to the 3d building data in certain cities, to provide accurate worldwide data for a flight simulation application. Since Google Earth uses 70+ TB, distribution would have to be limited to relatively small areas, or the flight simulator would have to be constantly communicating with Google Earth if the bandwidth would be sufficient.
It won't be long, though, until HDD's are large enough to store the whole thing (they're up to a TB now, and it was just a few years ago that 1 GB was a huge harddrive), although media to distribute the program in might be a different matter (blu-ray is only 25 GB, and we don't even really have it yet).
Algorithms should probably be used to sharpen the images for very low flying, and perhaps create tree structures where there appears to be vegetation, extrapolate houses and buildings to 3d, and simulate water where there appears to be lakes, and try to extrapolate height data for hills and mountains.