China loaned out Pandas to American Zoos to aid in breeding. They have now been able to find successful ways to breed Pandas, to the point where the Chinese government is somewhat reluctant to send more pandas to American Zoos. I think the best time for Disney to have acquired Panda was at the beginning of Animal Kingdom's existence, but my understanding was that at that time, they were too worried about the potential bad press they would receive in the event that they had a Panda illness or death.
At this point, I would love to see a second Asia walk through that include Pandas and Orangutans as the centerpiece animals and perhaps Red Pandas and Snow Leopards as complimentary animals. That would put Disney on par with any zoo in the world in terms of noteworthy species, and if you add in the presentation of these animals you'd have to consider it one of the best zoos in the world.
Some things that I've heard, first off - any Panda that's in this country is on loan from China, as of right now there is no chance of acquiring a Panda without working with the Chinese government. The Chinese government has been incredibly strict and specific with what they want for enclosure specifications for Pandas. Given the high benchmark for enclosures that Disney has already established I can only assume that there is some other setback preventing the acquisition of Pandas, it might be that Disney hasn't expressed interest, it might be that China is looking for another park and that Pandas to the Animal Kingdom are part of that agreement, or it might be that the Chinese government is flat out not releasing any more Pandas into American zoos because they seemed to have established successful Panda nurseries in China.
As for the Panda diet. Disney does have quite a bit of Bamboo on property, but they wouldn't be able to be self sustaining in feeding Pandas. Pandas it a wide variety of Bamboo, and that's all they eat. I'm fairly certain that all domestic zoos get shipments of Bamboo for their Pandas, they don't grow it on sight.