Sebastien: You can work with inkscape's default units (px) and export your artwork at 90dpi from the png exporter and you'll get 1:1 conversion of vector pixels to raster pixels. The same applies to the image imports, inkscape will take the real pixels and place them on the work area using 90 dpi.

Of course the feature that you're proposing is interesting and would be very helpful in several situations. This has been already discussed and if I remember correcly, there is a similar suggestion in the wishlist. Could you please check in the bug tracker if it has been already reported? If not, you can create a detailed feature request.

Gez.