A dirty sensor definitely will not cause lack of sharpness. I clean the sensor before every shoot, at least with a rocket blower. (I don't have any of the newer models that have sensor shakers.)
> I did get stuck shooting in the rain the other day and also wonder if moisture might have created a problem.
I seriously doubt it.
> I usually create a cover when I need to shoot in the rain, but got taken by surprise this time.
My advice about this is to get the rain cover or two that's necessary (I have a ThinkTank HydroPhobia for camera and regular lenses and an AquaTech for my 400/f2.8) and just put them in the bag with the gear. They're expensive (I probably have $400 in those two covers) but that's nothing compared to a trip to Nikon for a D3 and 400/f2.8. Especially these days, with Nikon's repair prices jacked up by 30-100%. I'd rather give the money to ThinkTank than to Nikon.
> Anybody else ever experience an unexplainable loss in image quality? Any advice?
It could just be drift, but more likely it's a mechanical problem - that's why many (lots?) of pros send cameras in for a CLA every once in a while.
But every time I've had this, the problem has been behind the viewfinder.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!