CO has always given me credit for cancelled flights - no questions asked. I used miles to upgrade on US Air recently and the flight was cancelled. The flight I was booked on did not have first available. I called the US Air service center and they promptly credited my account with the miles. Can't speak for other airlines. Had flight cancelled on UA a few weeks ago. Haven't seen mileage for it. A call to Mileage Plus WILL be placed.