I think the switches are fairly stiff, it stops you accidentally switching your camera on / off. The question is how stiff is stiff? I would imagine all models have the same stiffness, so visit a shop and see what the switches are like on the display Nikons. If you are still worried, take it to a shop and ask them for an opinion - I don't know how much a replacement switch would cost, but it could be more economical long term to get a 2nd hand D90 with lower shutter count than send a camera away to have the switch replaced.