Best guess is that this celebration will end September 30, 2006.
There are rumblings that the next celebration, WDW's 35th anniversary, will begin the next day.
The dining plan doesn't appear to be related to the celebration. It's a financial add-on. The jury is still out on Magical Express. Speculation ranges from it staying as-is to it going away altogether, with the reality probably closer to there being a small charge applied for it, if even that.