Ok so I just received a D300 and when I look through the viewfinder there are like 4 very light grey dots almost in each corner. Funny thing is I am a previous owner of another D300 and I don't remember those being there.
I am dealing with this myself. Had about 20 spots on sensor--some dust, some tiny lubricant spots. Got my Giotto's Rocket Blower today, and it blew about 90% of the stuff off the sensor. I have an Eclipse sensor cleaning kit from Copper Hill on the way, with which I will clean off the oil spots.
If you go to copperhillimages.com you will become an expert on sensor cleaning by reading their information.
Greg, Dust in the viewfinder is sometimes on the underside of the focusing screen, pointing down toward the mirror. If so, it can be blown off easily--you may have already gotten rid of it. Just make sure you point the camera with the open lens mount area facing down, so that the dust doesn't fall down by the shutter blades or sensor. If the dust is on the top side of the focusing screen, there wouldn't be much you could do but remove the focusing screen and clean it.
Occasionally I have had dust on the mirror surface, which could be visible in the finder. The best way to get dust off the mirror is to use a blower. You can sweep the mirror with a very soft brush, but be careful not to put any pressure on it, and never use anything wet to clean it.
As Brian said, dust on the sensor would not be visible in the viewfinder, only in pictures.
In a DSLR, the imaging sensor is located at the rear of the camera behind the reflex mirror and mechanical shutter. When you're looking through the viewfinder, you see the scene as projected onto a focusing screen above the mirror - the sensor is not in the light path at that point so it's not possible for spots on the sensor to be seen in the viewfinder. When you press the shutter, the mirror lifts (thus blocking the viewfinder instantaneously) and the shutter opens, exposing the sensor to the light.
Any spots you see in the viewfinder will be on either the mirror or the focusing screen (they are unlikely to be inside the prism housing if they moved when you used your blower).