As someone who studies hurricanes a little, maybe I can add a few points to think about...
1. First of all, it is still too early to tell anything for sure, the rest of my points assume that Ivan will stay on it's current forecast track as seen on the map linked below.
Forecast track
2. If Ivan stays on that track, it should affect WDW during the early morning hours of Tuedsay, Sept. 14th. My guess under this scenario is that the parks would close early on the 13th and probably reopen on the 15th.
3. Ivan shouldn't be as powerful as Charley or Frances when it reaches the WDW area. If the current forecast holds, Ivan would have been over land for nearly 24 hours by the time it gets as far north as WDW. That is alot of time in the life of a hurricane. The land interaction will have a weakening affect on Ivan. Even if Ivan is a category 4 storm when it hits South Florida, it would surely be no stronger than a category 2 by the time it gets to WDW - and probably weaker.
4. Ivan will probably have the forward speed of Charley. Frances was such a problem because she was so slow. Unless things change, it appears that the atmospheric conditions are going to move Ivan pretty quickly once it hits land.
Again, don't take any of this as gospel. These are just some things to think about.
A good web site for keeping track of the storm is
Moreweather.com