Hi John - here is how i told my little guy last week:
We leave for the World in 20 days and tried my best to hold off telling my 8 year old DS he news, but couldn't wait any longer.
Started off the day yesterday by adding Disney items to his "Treasure Chest" - first a Hidden Mickey book, followed by a lanyard and
lastly 20 pins that I bought from Ebay for $20! Everytime I had the chance, i would drop another one into the chest - it was priceless. He is such a HUGE believer in Disney magic and was/is convinved that Tink was adding these items.
Fast forward to the evening when we returned home from a Birthday party and a mysterious clue was in his treasure chest and the hunt for additional clues began
Take a look at the video I posted to see how it all unraveled (it's a bit long - with the actual suprise piece can be found at the 9 minute mark)
http://www.youtube.com/watch?v=Fj9lQoarP0A