Grand Floridian Cafe has great desserts, try the sampler. The fondue, while simple, is also excellent. We got a tiramisu, cheesecake, key lime pie, and chocolate cake portion. All of them were delicious and well presented.
California Grill desserts change frequently and are very seasonal, so don't plan to walk in there with your heart set on anything. I've never had a bad dessert there, though I second the vote for the warm Valhrona cake.
Le Cellier- the dessert sampler was outstanding, and had the Maple Creme Brulee, chocolate whisky cake and a really excellent house made sorbet.
I second the vote at Liberty Tree Tavern for the butter griddled poundcake with pecan caramel sauce. It was heavenly.