I know it's a bit early, but Screamscape has already heard something:
Sounds cool if it happens.
Resort News - (11/5/07) As we move closer to a likely 16 month long WDW 40th Anniversary celebration (Oct 1st, 2011), we’ve heard an early rumor suggesting that they may celebrate by promoting a major new attraction to each of the four parks, similar to how things went down in 2005/2006 when they added Everest, Lights Motor Action, and Soaring. In the meantime we’ve heard that Disney may also have drawn up plans to add a major new attraction to both Typhoon Lagoon and Blizzard Beach over the next couple of years. Anyone know more?
Sounds cool if it happens.