I've read the whole thread and am confused. I guess I don't understand the OP's issue.
The contract reads that for any date of arrival after 12/31/08 (that is, any arrival date in 2009 and beyond), at this point your rate is not guaranteed. The only thing that is assured is that you'll have a room -- but the price for that room is yet to be determined. If your date of arrival is in 2008, it reads that your room and rate will not change, so long as you don't change your reservation. 2008 rates have been released. So, what is the concern? (As a technical matter, there is no enforceable contract if there is no price, as price is the key term of any contract. So, Disney is going a step further than they need to and saying that, even though there really is not contract because there is no price yet, we'll promise to have a room for you anyway).
The rest of the contract refers to a situation wherein you received a rate you weren't entitled to. A postcard rate is an example. Another is a
travel agent or airline rate. If you aren't entitled to the rate you booked, Disney reserves the right to take any remedial action is wants as a result of that -- re-pricing, cancellation etc. The language about mistake covers Disney for the argument a person might make that they thought they were entitled to use the postcard rate and just made an innocent error. What Disney is saying is, if you're not entitled to the rate you have, they don't care if you got that rate by an intentional fraud or a stupid mistake. You can't keep it.
Getting to contract law, contracts from Disney regarding hotels are "contracts of adhesion". This means that Disney writes the contract and you have no right or ability to negotiate the terms. You either accept the contract to get the room, or you don't accept the contract and you don't get the room. The contract whenever you park your car in a paid lot is the same -- park your car and accept the terms, or don't park your car.
Contracts of adhesion receive particular scrutiny in any court case because judges understand the lack of bargaining power in one party to the contract. Nonetheless, contracts of adhesion are generally enforceable, although a judge may rewrite one to correct obvious inequities. But to have that happen, you have to sue and go to court. Disney, as the writer of the contract, is not going to re-write it because someone doesn't like it.
Contract law is a state law matter (it is not a U.S. law matter). Contract enforceability, while hard for a layperson to understand, is always an issue in any lawsuit concerning a contract. Contracts can be unenforceable for any number of reasons. Mistake, duress, incapacity, the contract is against public policy, etc.
Again, I'm missing the OP's issue, I guess.