The Millineum Celebration ran from 10/99 through 12/00.
100 Years was supposed to run the same way, 10/01 through 12/02, however, it was extended for two months due to the slow start brought on by the effects of 9/11.
So there was a 9 month gap between MC and 100 Years.
Most speculation is that that next "big" celebration will be for DL's 50th, but it will be labeled as more of a "50 years of Disney Parks" rather than "DL's 50th birthday".
IF that is true, it will probably start towards the end of 2004, and run through 2005.
It would be difficult to squeeze in anything significant during the 19 months or so between 100 Years and 50 Years, so anything that happens will probably be minor. (not that there was much real substance for 100 years anyway...)