I used the Lowpro Orion Trekker II through Disney and Universal.
http://products.lowepro.com/product/Orion-Trekker II,1989.htm
Top half of the bag is for general stuff, books, maybe large filters etc. water bottles (can just about get a magazine in it, but not a laptop). If you do put something heavy in the top half, don't leave the zips in the middle at the top, they sometimes pull open (rare, but of concern in crowds).
Bottom half has both a zip, and a catch mechanism. Either is enough to hold it shut with my camera gear in (which I did trek around animal kingdom with). It has had 1 Canon 400D, + up to 5 lenses (well, ok, one of these is actually a video camera, but it is of the same dimensions basically as my 28-135 lens). If I have the 70-300mm on the camera (my longest at 17cm), then I can get 3 other lenses into the bottom OK, as the panels are movable. If it is off the camera, the height of the bottom section is enough so that it can be stored vertically. The front pockets can store sunglasses/phones or mini-tripods OK I find (well, actually, my sunglasses got crushed/broken last year, but I think that was more the fault of bag being thrown out of the car)
Going through airport security has never been a problem with it. I just flap the bottom open and it goes through the X-ray machine. If you are happy walking along on just a catch, it can be very quick to get into (I do on coastal/walks, but not usually in crowds), the zips don't really slow down access that much. If you are into serious walking as well, it has both a chest-strap and waist strap.
The camera stores can be very hit and miss. I have not had any success with the off-the-road stores. They all seemed a little, seedy (Hmm, so you have just dropped $2000 off the price for me, becuase I said I wasn't sure? Once?)
In the Buena-Vista outlet village there was a semi-decent one, and also in the Floria Mall complex there were a couple of shops.
I got my Lexar CF cards from Walmart at good prices, however, the prices do vary depending which Walmart you are at ($10-$15, which given the cost of petrol [must be at least $3 now] and the exchange rate, you might consider shopping around).
I use the packhorse, sorry the wife, for carrying my large tripod or monopod if we go somewhere where it might be necassary.