Comment 1 for bug 362995

You have to save and re-open the document to see your units choice propagated to all the UI selectors.
If you want to always use inches save a blank document with inches as default units as a template.
The templates are stored in the "templates" folder, inside the configuration folder (it depends on what operating system you're using. In linux is usually .inkscape; in Windows is somewhere in the Documents and Settings folder, but I don't remember where).
The default.{your locale}.svg template will be loaded everytime you open inkscape, with the chosen page properties.