Sorry JT, nothing in your reply is true in any way. Even with pockets the size of Cameron's, budgets still need to be met. No one is going to bank roll a $500 mil film (not even Mr. Deep Pockets himself). That problem is amplified by the current state of the industry and bombs like John Carter.
The thing is, they're working new technology to bring the costs down. Yes, anything is possible. But it isn't possible to do anything you want without spending a ton of cash. ILM is working feverishly on the solution in order for Lucas to be able to bring his new live-action show to tv. From what I've heard, there are over 200 scripts written and they're just waiting for costs to come down. The tech is coming, it just isn't here yet.
So, if you're James Cameron, do you dig into your wallet to shell out the extra $200 mil or do you wait a couple of years when it'll be cheaper and virtually no risk to you personally. Considering you can use those couple of extra years to continue to develop the story, characters, design, etc., it's an easy choice. You wait.
As for FotLK, if you're TDO, you go ahead and move forward with relocating it. It doesn't fit where it is, and you're now ready to move on a moment's notice if and when they decide to green light Avatar (or the land is ready for a different attraction if Avatar gets cancelled). I'd bet the money is already in the budget anyway.
Either way, if you're Imagineering, you get concepts up and ready for TDO to look at in case the $500 mil they have ear-marked for Avatar is burning a hole in their pocket. My guess is that the money is already on the books, and TDO will want to spend it before it's yanked out from under them. They'll figure out how to pay for Avatar in five years, when it actually makes more sense to build it (or, they don't care because it will be someone else's problem to deal with).