My 4x5 was a Crown Graphic. I really didn't worry about it getting wet, but usually ended up shooting it from under the opened lift-gate of my Cherokee. Got me out of the wind, too.
For 35mm, I cover my tripod-mounted camera outfit with a hotel showercap. Those are great. When taking the picture, I just pull it off for the exposure. A little water won't bother the rig. It's plenty tight. The "Ti" versions of the OM line are weather-sealed.
If conditions are extreme, I'm using my Di*ital Olympus E-1 which is one-step short of being a Nikonos.