Nikon have never 'guaranteed' thier products are weatherproof, but they are: I've used F5, D1, D1x, D2x, D3 and D3x cameras with a wide variety of lenses in all conditions over a period of many years: rain, sleet, snow, dust, heat, humidity, from the Arctic in Norway to the Jungles of Belize and the Deserts of Iraq and Afghanistan - all with no problems.
I even fell in a river carrying a D2x and D2xs with 17-35 and 70-200 VR lenses and dunked both of them completely underwater. Both cameras were dried with a damp hankie and carried on working with no adverse affects.
The latest range of Pro lenses with the little rubber gaskets provide a little additional dust barrier for the sensor but that's all...the rest of the seals were already good enough as they were.
Light rain, drizzle, even torrential downpours (ask the TP guys who were with me in a trip to London a few years back if I looked even slightly worried about the cloudburst that hit us in Trafalgar Square!) should pose little or no problems apart from misting-up of the eyepiece and raindrops on the front element. Carry a fake chamois or something equally absorbent like a good micro-fibre cloth and keep wiping it away...
If you're doing a static sports shoot or something similar then a rain-cape might work for you...personally I've never used one even though I have the Kata ones issued to me...