I just finished printing off our scavenger hunt clues! We're going back in January, and our kids ask almost daily when we can go back (the last time was 3 years ago) I had thought about surprising them that day, but I'm like the OP, I want them to have the anticipation, too. So, we're telling them in a few weeks, and the rest of the time before we leave will be family planning, re-living our last rip, etc. I'm so excited!!
Anyhoo...I'm planning on the scavenger hunt. I have 8 envelopes with clues, with the last envelope containing separate "airline tickets" for each that I printed off from the Creative DISigns board. They're so cute, and have each kids name on them (as well as mom and dad and grandpa & grandma). Also the date we're going, where we're staying, etc. Here's a link to that thread if you want to check them out. Telynn does a great job!!!
http://www.disboards.com/showthread.php?t=1949606
After they open the last envelope, I plan on putting some Disney music on, and I have some older Passporters I bought for each, to open. ( I wrapped them in Mickey Mouse Christmas paper, since our trip is their main Christmas present. I just couldn't wait until Christmas to tell them!!) Then I hope to watch Disney movies, go through their passporters with them (the 2007 passporters were only $1 each on their website, so I got one for each kid, even though the 4 year old can't read...he'd still want one to be part of the group!) and just live in the excitement!!
So that's how we're going to try it! Have fun however you decide to tell your kiddos!!!!