Much like the PP, it comes in stages for me. It starts when I am on the Disney website, getting the prices. It ratchets up a notch when I actually make the reservation. There is a bit more excitement when the invoice comes in the mail. Making our ADR's brings it home a bit more, then, when I make the last payment, and it is paid for it seems a bit more real.
The next tick in the excitement comes when we get the package in the mail with the vouchers, and luggage tags in it. It becomes a bit more real when my name goes up on the calendar at work for the time off. Packing up for the trip is almost unbearable, it's so close, yet we still have to wait, and it isn't quite a reality yet.
The 20 hour drive from New York to Disney World is fueled 100% by excitement. Driving under that Welcome sign is huge, but I think that the biggest "It's Real" moment is hearing the CM at the hotel say "Welcome Home."