Dust is a big problem with digital SLR's, with the D70 being particularly prone to problems. The problem is enhanced by changing lenses in dusty environments, but finding an environment that isn't dust filled is damn near impossible. Even without changing lenses at regular intervals, dust can get onto the CCD leaving your images with a nasty splodge.
My D70 suddenly collected a number of dust spots all over the CCD when I went on holiday to Bulgaria skiing. As I hadn't changed the lenses for over 4 months the dust was a surprise. My bother also suffered a similar fate when his D70 experienced a sudden temperature change. So it seems possible that just the atmosphere which the camera is in can lead to an increase in the dust collection. Also the CCD's charge will attract dust, so coupled with temperature fluctuation, the problem of dust is unavoidable.
However, the problem can be rectified. My bother cleaned my CCD with a contact wipe, a soft swab that is dragged over the CCD to remove the dust particles. Other methods can use an air blower to remove the dust but may be ineffective if it's particularly well adhered to the sensor. Also, using an aerosol compressed air blower can be severely damaging if any liquid leaks onto the sensor.
The CCD's appear quite robust to light contact as they have some protective shield over them, however care must be taken and it's advisable to only clean when the dust gets to be a big problem. If dust is in the lower third it is likely to be absorbed by ground in your shots and so could be left - sky spots are more visible and annoying.
The following two comparisons show a before and after shot of the dust on my sensor which is now clean and dust free :)
Central dust spot
Same spot post cleaning
Top corner speck
Now clean :)