Here's what myself and most other wildlife photographers do. We use clear plastic bags. Ideally, the kind you get in the vegetable section at your grocery store. They are light, pack in your bag easy, and are actually fairly strong. Wrap it around your camera and lense with the top of the bag at the end of your lens. You then tape around the end of your lens to keep it on tight. (or tie it off in a pinch) Ideally, you always have a lens shade so that gives you the protection so that the front of your lens does not get wet. These bags are light weight and you can use all your buttons and see through the bag to get to all the buttons on the camera. Also, you should be using a tripod so you take it off the quick release or unscrew your lense/camera before you put the bag on, then just thread it back on through the bag. It's not high tech but it's the best solution.
Oh yeah, my gear is now weather resistant, but it was not back in the old days. Weather resistant/proof or not, I'm still using the bag. All Canon L series lenses are weather resistant/proof.
shawn
