A counterpoint.
This is just the first step in airlines replacing per ticket commissions with incentive-based commissions. The corporate travel agencies are going to continue to do business as before for corporate travellers.
Expedia and Travelocity cater primarily to low-fare pleasure/business travel. These new developments give them an incentive to increase the volume of sales with each airline and hence overall so that everyone benefits. Note that these online travel agents were already doing preferential treatment for certain carriers with whom they had promotional arrangements. Why then should others give them commissions for a product that sold itself despite their bias?
Having used Travelocity and Expedia extensively, they are hardly innocent bystanders here in they way they manipulated the availability of seats from different carriers. Sometimes I have had to force a carrier in advanced search to unearth lower prices from that carrier.
I predict that each of these airlines will work out a performance-based system like Northwest. Different airlines are in different negotiation phases.
Airlines with the best routing will still have some power over these online agencies. If these agencies were only able to provide ATA and National fares in their search, they would seriously lose their business.
I have no afficiliation with any airline and fly at the cheapest fares I can get and don't think any airline is a saint. But I also recognize that airlines need to increase their margins now and with the tickets going electronic in most circumstances it makes no sense to support a distribution infrastructure that is rather inefficient in majority of the sales.