I'm in the same boat. Our trip is in January, and we haven't told DD7 and DD2. We are planning to tell DD7 and DD2 at Christmas with WDW for Kids by Kids book for DD7 and the Sing Along DVD for DD2.
We talk about it in that "what if" tone, and DD7 has not picked up! She even saw my new copy of the Unofficial Guide 2008, and she just thinks "Mommy likes to read about WDW", which is true. She hasn't seemed to put 2 and 2 together. I talk about restaurants that we might want to try with DH in front of her, and I have even asked her about restaurants (all ADRs are set). We still watch the planning DVD and the Travel Channel WDW shows, so the excitement still seems there. It is as if she knows that we are going in the future (and that's what we lead her to believe), but not that it is actually planned. I'm just wondering if she'll figure it out before we actually tell her. Right now, we talk about a trip hypothetically.
I'm dying to tell her, but it is just a lot of time for her to wait. We went last January, and she knew from the March before. It just seemed so long for her. I would wait until November at least. Talking to her in "what ifs" seems to work for me and, hopefully, her. She's excited about going again in any case.
(I'm afraid to put a countdown ticker in my siggy just in case she's looking over my shoulder!)