The existence of this liability is what caused many airlines several years ago to change their programs so that miles expire (although that change didn't really stick). This is also why airlines can be tempted to devalue awards. Increasing miles required for particular awards can reduce the liability significantly and immediately. It will be interesting to see if the 2003 redemptions end up being much higher than 2000-02. Anecdotal evidence would suggest a significant increase in redemptions and attendant drop in revenue per seat mile.