Well, Dumbo's Circus is part of Phase 1 of the Expansion. So it will open in 2012. The castle walls are also part of Phase 1 which means they will open along with Dumbo and the rest of Phase 1. The castle walls are housed where Dumbo currently is located, therefore having Dumbo being demolished and making tourists wait for the Expansion to wait. I'm not sure if i'm making my point clear...
EDIT:
Phase I: Cinderella Area, Belle Area, Ariel Area, Dumbo's Circus, Castle Walls, new Winnie the Pooh area, new Train Station, new Barnstormer as a part of Dumbo's Circus.
Phase II: Pixie Hollow(imagineers hinted towards an attraction for this area).
Still questionable: New teacups area, Casey Jr.