Are you currently planning to go next year? If so, then get her involved sorta. Buy or print out your own "Disney Themed" calendar and start a count down. If you have a scanner, let her choose some pics from your trip and print your own calendar. If you dont have a scanner, take the pics to a copy store or print shop and have them make a calendar. They seem to run around $20 and I've seen them offered at WalGreens and WalMarts. Then with a red marker countdown the days. Maybe on special days like 100 days to go have a Disney breakfast and make ºoº pancakes and name all the other food with Disney names. Add lil notes to the calendar like on the last month to make a paperchain to count down the days too. Let her ask for Disney $$$ for Bdays and holidays or tell relatives she's really into Disney. Some of this may be over her head but you'd be amazed at what kids can comprehend when its something they are obessed about.
My dad reminded me of that a couple weeks ago.....seems when I was 3 and we were on a 6 week vacation "outwest" I start worrying about finding home again and about counting and saving everyone's change to buy things with. Seems I learned to find the highways and destinations in the atlas and figured out when he would pass cars when it was ok for him to get back in the other lane. He said I was a whiz at counting change and knowing what my change should be when I paid for stuff myself.