Inkscape uses a internal default of 90dpi. That is sad news for anyone who uses mm as everything get's these nice values of 3.45etc px/mm
I guess it has to do with this property. I once suggested a setting in Inkscape to define the amount of pixels per unit by the user rather than 90px/Inch so you can more easily work in mm or whatever units, but they didn't quite get the point. If you can post your issue on launchpad as a bug (same as feature request). I can post on launchpad from Chongqing China, so my wish list never gets there
