We surprise them for 2 reasons...
1. If we told them in advance, the wait would be unbearable for them (and us as a result)
2. If something happened and we had to cancel, they would be devastated. I would rather them not know to begin with
We surprised them for their first trip to Disney in July 2008, for oldest DD's 7th bday. She opened her gift, a Hannah Montana suitcase. Inside was a note telling her to get it packed, because we were heading to Disney the next day. She was super excited, but I don't think the twins really knew what on earth we were talking about.
Well ever since that trip my kids have done nothing but ask when we are going back -- we all LOVED it! So we told them in July 2010 (we hope to go every other year in late July, when swim team season is over for older DD and value season starts at our fave resort). That was our plan, but I was able to snag ressies over spring break (mid March) on the buy 4, get 3 promo, so even tho it's peak season we will go for the same price as we did in late summer.
They know we are going next year, but they think we're not going until July. However, we are going to pick them up from school that Friday before Spring Break and tell them we are leaving THAT NIGHT for Disney. They are going to freak out, no doubt!