It's more like a dithering problem like webtrekker said.
If you print through VPM it is error diffusion dithering, which gives very bad results in clear skies and white / grey backgrounds (banding appears).
I've printed the same document through VPM and directly from Acrobat (where I changed the dithering). The dots are gone, but now I have e checkboard pattern.
I'm going to test this a bit further and let you know if I find something.