Beginning May 5, the Magical Express will be available to take you from the airport to any WDW resort for free. I have read on DC that SW is participating but you'll want to check. There have been some negative comments about Mears on DC, mostly involving the long wait. We opted to rent a car, with discounts it was about the same price as a transfer and we'll have a car available all week if we need it. Check mousesavers.com for information on transfers as well as rental car discounts. A great source for money saving tips.
The Unofficial Guide has an estimated attendance that many DCers have sworn by:
http://www.touringplans.com/BestDays.htm. The Unofficial Guide book is a great for planning your trip.
Have fun!