I’m inclined to recommend the Nikon D90 over the D300 to most people. Unless you are using the camera to hammer nails, the metal chassis of the D300 is mostly overkill (it feels great though ;0) and the weatherproofing (which I did buy into) is mostly marketing hype. There are reasons to buy a D300 over the D90 but if you have doubts about the toughness and capabilities of the Nikon D90, watch this video shot while speed climbing the North Face of the Eiger:
Bear in mind that the whole thing, movie and stills, were shot using the D90. To watch it in High Definition, Click Here.