Effective today, American Airlines AAdvantage accounts must have a qualifying activity at least every 18 months, otherwise the miles will cease to exist. From the AA website:
"AAdvantage members now must have mileage earning or redemption activity once every eighteen (18) months in order to remain active and retain their miles. Effective December 15, 2007, mileage balances will expire from AAdvantage accounts that have not had miles either earned or redeemed within the previous eighteen (18) month period. If the last activity in your AAdvantage account was prior to June 15, 2006, all miles subject to expiration will expire on December 15, 2007."