Eric has given some good advice. As someone who often has to shoot in the rain, covering sports, I would only add that even a little bit of protection is better than none. Shooting from atop a monopod, in light rain or mist I'll sometimes just drape a towel over the top of the camera and lens, covering most of it. Even that little bit of protection keeps water from building up on the camera surface where it can find its way through the button and dial ports.
And if all else fails, a plastic garbage bag makes for an effective emergency rain cover. Just punch a hole in the closed end and stick the end of the lens through.