The anniversary of the Magic Kingdom at WDW is Oct 1, 2006. I haven't heard anything at this point, but I would assume that there will be some merchandise. The Orlando Sentinel said something today about a new parade, but that starts soon.
Disney has already announced that their next marketing campaign will be a global campaign the "Year of a Million Dreams" with the message that the Disney parks are "Where Dreams Come True." So don't expect a major marketing campaign based on the Magic Kingdom's 35th or Epcot's 25th anniversary. The anniversary messages may be woven into the "Year of a Million Dreams" at some point, but I wouldn't expect anything big.