I don't use a specialized camera pack, just a normal backpack with some lens and camera cases in it to sort of crudely make it into a camera bag. ; But it has a built in rain fly that tucks into a pocket in the bottom. ; I've made use of it in Disney World, it works quite well.
The backpack itself is water resistant enough to take a brief shower, or at least to hold off the elements until I can make it to cover. ; Then for the Florida style torrential downpours I let the rain fly keep my stuff safe. ; I wouldn't go on a trip like that without a rain fly now, it's become part of the resources I'm accustomed to having at my disposal. ; It's nice to be able to shrug off a rainstorm and keep moving.
I know I've seen similar rain covers being sold separately. ; Here, check this list out:
http://www.rei.com/search?query=rain+cover&button.x=0&button.y=0
Some non backpack related stuff snuck into that query, but yeah, such things can be found if you look for them. ; I'm not sure about all of them, some seem to completely cover the pack, but others clearly allow access to the straps so you can wear the pack while it's covered. ; The prices seem a little high, but REI prices tend to be that way. ; Check out your local outdoor sports megastore if you have something like that around and see what they've got.
Incidentally, last year I discovered that my backpack also protects my gear from beer.
Eh.. was at an outdoor festival sort of thing. ; An event where you're not allowed to bring in water unless it's a factory sealed water bottle (no, I don't buy bottled water), presumably to keep the teenagers from sneaking in alcohol (and also to force people to buy the overpriced beer being sold at the festival), but where I apparently can't avoid having some slob slosh beer over my backpack.
Happily all I had to do was empty it out and hose it down. ; It probably ended up cleaner than before it had been sloshed with beer, that pack and I have a bit of a history and it was probably due for a hose down.