I just returned from DisneyWorld Orlando last Thursday. I was there for 7 days and like a previous poster, I used the Lowepro Slingshot 200. I carried the 20D, 24-70mm, 10-22mm, 50 1.4, extra battery (that I didn't need) and CF cards, and kept snacks, napkins, tissues in the top pouch. I carried it from 9:00am until 9:00-11:00pm every day, taking it off to eat or when on rides. And I herniated a disc (L4/L5) this past winter and was worried about aggravating it, but it wasn't a problem. The bag was comfortable and held what I needed. It was also very convenient to "sling" it around and grab the camera to not miss a shot.
It was a bit less convenient getting into the parks because under an "orange" security state, they're searching every bag when you enter the parks. So you have to remove it and open it all up. You get good at that after a while!