I'm one of those people who really hates it when the vacation ends, I get pretty bummed about it. If I have to pay for it when the bloom is off the rose, so to speak, it just makes that feeling worse. So yes, I pre-pay, though not long beforehand.
I know how much the car rental will cost, for instance, so I mail in that amount the day we leave. I also know from past experience just about how much we will spend on meals, so I often add that to the check, too. Usually the room is free for us (Starwood points), and the airline tickets would have been purchased well in advance. We also always buy larger passes than we use, and save the days for a second trip.
BTW, Beansmom, I don't think the question was about putting a balance on the card so that you could charge big-ticket items. I think that the question was about paying for the whole trip before you take it, so that you don't get a large CC bill after you come home.