we've been so many times, and the kids have always known and helped plan, shop, pack, etc... I definitely love all the anticipation and sharing it w/ the kids, and they love it too.
we haven't been in a year, and we were thinking of going and talking about if/when in front of the kids (they're 6 and 8), so they were of course begging to go... as the day went on, i whispered to dh that we should surprise them this once since they've been so many times and have been in on the planning... so at dinner they ask if we've decided when we're going, and i start saying "well, we're not sure we can go..."
DS6 starts crying and runs upstairs to his room, dd8 starts crying (but stays at the table)... so that kind of blew that... I told them we were going, but we want to surprise them, so we'll keep "when" a secret, and they're thrilled! They get to plan, I get to talk about it as much as I want

(just not the dates), but they know one morning in the summer we'll wake them and say 'get in the car, we're going to disney!'
This is the best of both worlds for us!
(disclaimer... we're doing
a lot of vacations this year, so the kids aren't anticipating just this one trip - they know about all the rest... otherwise they might drive us NUTS w/ the when... so this might not work for everyone).