I told myself I would stick a fork in my eye before I'd do another kids' movie post but something Nathan said forced my hand.
Nathan pointed out that the animals in Babe don't so much talk as communicate. When my dog jumps on my cat and begins sodomizing him, it's clear some communication is going on. In Babe, we get the communication translated to human speech. Considering that I based a novel on this, I'm in full agreement with him.
I hadn't considered it but (usually) the better "talking" animal seem to do this but the horrible ones restrict the power of speech to good animals (evil equaling carnivorous).
The Disney movie Dinosaur has one of the most amazing opening sequences I've ever seen, an incredible blend of CGI dinosaurs and real landscapes. Then they start talking and everything falls apart. The evil raptors and carnotaurs simply grunt and roar. They might be delegated as stupid brutes but they're the only dinosaurs in the rest of the movie that seem real. This is usually true in the Land Before Time series (unless the cute baby dinosaurs meet a cute baby T-rex or a singing crocodile).
In the Jungle Book, the tiger talks. In Tarzan the leopard doesn't. In the Jungle Book, the vultures talk. In Lion King, they don't.
Of course, how low do you go? Should fleas talk? Should a virus? Might be interesting to try.