I have no idea if they've run Epcot commercials overseas. Regardless, it doesn't prove whether or not they worked.
On the marketing campaigns, I also expect something to start in Spring '07, or Spring '08 at the latest. They've told us they expect to primarily drive growth through marketing, so it follows that we'll see another celebration within a year or year and a half after the current one ends.
The danger, however, is that the public starts to be less impressed by another "celebration" because they become too common. That would be the only thing that would delay another one.
Regardless, I am positive that the theme of the next celebration will be primarily driven by timing and the broadness of its appeal.
In other words, they will first decide (or have already decided) that they want a celebration to start on a given date. They will then look for the opportunity that has the broadest appeal, not necessarily the greatest significance.
The question is, do they like the way the "50 years" celebration that includes both CA and FLA has worked out both in terms of public response and reduced marketing costs? If so, look for something that allows them to tie them in together again. In that case, an Epcot celebration would seem unlikely.
But if they want to go back to more resort specific marketing campaigns, then Epcot probably fits WDW pretty well from a timing perspective. I think they'd prefer to have something that is less focused on a single park, but Epcot still might end up being their preferred choice.