Apple and ABC have posted another television show to the iTunes Music Store today. Commander in Chief is now available on iTunes.
NBC has also promised that new shows would be added along the way.
Meanwhile, there are reports that Microsoft is not taking Apple's success lightly and plans on launching a subscription-based [Movie]* download service. The service would presumably offer either unlimited downloads or one with a high limit for a flat monthly fee similar to Napster's present music subscription service.
* Update: Movie service, not TV.