Some of the restaurants at WDW are owned and operated by Disney itself, while others are owned and operated by outside companies. For those restaurants that are not run by Disney, they each decide independently whether they want to participate in the Dining Plan. Some do, but many don't.
If you click the link found in the question
HERE, you'll be taken to a downloadable pdf of the restaurants participating in the Disney Dining Plan in 2009. You are correct that neither the Yak & Yeti in the Animal Kingdom nor Nine Dragons at Epcot are on the plan next year. This is because these restaurants are not Disney owned and they chose not to participate in the plan. This is also why almost none of the Downtown Disney restaurants are on the plan; nearly all of them are non-Disney owned.
Restaurant participation in the Dining Plan is certainly a factor you should consider when trying to decide whether the Plan is right for you. If your favorites are not on the list, or you're planning to spend a lot of time at Downtown Disney, then it might make more sense for you to skip the dining plan and just pay for your meals as you go.