Comment 10 for bug 166970

Actually, Mercury, SVG 'px' are treated everywhere in inkscape as physical units (at 1/90", just like a pt is 1/72"), so the mixture of SVG px and mm is not a problem.

Your proposed solution I think makes lots of sense. If the user is designing icons or web pages, and wants 1 SVG px == 1 screen pixel, then they use the default setting of 90dpi. If instead they are doing print work and want 1 SVG px == 1/90" (and therefore 1 document mm to equal 1 mm on screen) then they can set the value to their actual screen dpi.

I think the only think inkscape would have to change is the way the zoom factor is calculated...