Weather-sealed and Weather-proof are two different things. It only takes one drop of water in the wrong place to cause issues. While most of the high end bodies are weather sealed, No manufacturer will warranty a camera or lens for water or moisture damage.
If you plan on shooting in adverse conditions I would recommend getting a quality rain cover like one of the Think Tank Photo Hydrophobia's. I use rain covers on all of my bodies including the single diget D series when shooting in the rain or snow. An ounce of prevention is worth a pound of cure.