http://disneyworld.disney.go.com/med...2010Dining.pdf

The cost of the Disney dining plan is $41.99 to $47.99 per night per adult (10 and older), and $11.99 to $12.99 per child (3 – 9). The Disney Dining plan is priced based on your travel dates, and is higher priced during the following peak times:

February 12 – April 10, 2010
June 4 – August 14, 2010
December 17 – 31, 2010

It appears as though the cost of the dining plan is based on the day you check in, so if you check in one a date when the Dining plan is the cheaper price, and your vacation carries over into one of the higher priced times, you will pay the lower price for your entire stay.