Last year (2009) Columbus Isle was closed from around August 25 or so until October. They said it was for renovation, but generally it's a time of the year when (in the past) they have had a number of hurricane problems and occupancy is not generally very high to begin with. So I'm guessing they may be doing the same thing again this year, and perhaps actually scheduling any necessary maintenance or renovations at the same time. But you could call and confirm what dates it will actually be closed.
Last year it closed with the French charter on the 1st of September. I know we re open on the 16th of October 2010 but it is a private group, so yes it re open officially on October 23.
Just made different simulations (August and September 2010) and the French site too says closed till October 23. They propose Les Boucaniers instead...
They're probably trying to gather GMs together in order not to keep all the Caribbean villages run with a low occupancy.