I'd like to give a special call out to U.S Airways, who not only charge $2 for a bottle of much needed water on the plane (tip to travelers: the water from the bathroom sink is not potable. Trust us on this one), they also have placed wonderful ads on the tray tables. What happens when a business reaches a point where it can't sell anything else? I guess it goes out of business. If your tray table ads and water sales are the last source of income keeping you afloat, though, you probably should rethink some things. What's next? Sponsored Security checkpoints ("Safety brought to you by Coke")? Reclining seats for $.25 per degree? Pay toilets? What's the next wave of air travel monetization?