
The bigger sibling Nikon D300 (see image above) has withstand hell but a more preventive approach should be adopted for the Nikon D90. [Fotosharp] recommends a "rain coat" and it looks interesting. While some others simply recommend a conventional zip-lock bag packed with silicon!
Well, I guess as long as its operating within the 0-40 Degree Celsius / 85% humidity boundary, you should be fine. Here's the [comment] from some users about their Nikon D90.
Basic Tips - Always remove your battery from the camera during cold transport and keep it close to your body (warm).