They'll charge you for a stale ham sandwich, they'll charge you for a paper ticket, they'll charge you for an overweight bag, and some airlines will certainly charge you to redeem your frequent flier miles (How Stuff Works page on frequent flier programs). Air Canada started this January charging a $25 fee to redeem frequent flier miles. In March American Experess started charging a fee per point (up to $50) for transfer of Membership Awards points to frequent flier miles. Diner's Club also tacks on $.95 for every 1,000 miles redeemed throught its Club Rewards program. Then there's the concern of airline bankrupcies: here's an article on what happens to your miles if the airline files for bankruptcy.