We did "the big reveal". When we got APs in December of 2009 we knew we would have a trip in October 2010, but decided if we could get cheap enough airfare in April 2010, we would go. DD spring break was 2 weeks after easter, so we only pulled for for 2 days, and went for only 4 days over a weekend.
Anyways, we didn't have to worry too much about the school issue since she was only missing 2 days. We didn't tell her until the morning of the trip. We had managed to pack and everything without her knowing. (It's pretty easy to fool small children as they are pretty oblivious to what's going on around them.)
I was a bit disappointed with her response. We had told her I was going to take the day off from work if she had been good at something the night before, and we would do something fun. We listed the typical things (zoo, aquarium) then we said "How about instead we go to Disney World?" Her response was kind of "Hmmm, really?" No jumping up and down. No screaming. I mean, she loved it, don't get me wrong, and she still talks about her surprise birthday trip. (I think she keeps hoping we'll do it again.) Just it was a very laid back response. I think she was much more excited counting down to it, and it was much more fun for us as parents...so truthfully I am not sad we did it, but I probably would never surprise her again.
That said, it is pretty priceless to spring it on the last minute. If I were you, I would try and keep it completely secret. At worst, have a "back-up" story: however, make sure the back-up story doesn't have nearly the "cool" factor that a Disney trip has. IF you tell them they are going to their favorite beach, and then switch to Disney World, you kid get initial disappointment out of them. ("What, we're not going to the beach?")
We also kept it a complete secret from EVERYONE...our friends, neighbors, only a few immediate family members knew. (Grandmas and Grandpas, and they all lived out of town.) This way we didn't have to worry about any slip-ups.
Hope that helps! If it's any concern...I promise I won't tell!
SkierPete