It's not easy. We had to do it once when we forked from Sodipodi and it was not a fun experience: there are a lot of places to change, and it's especially tricky because some instances of the application name should be left alone -- e.g. namespace URLs -- or the application will not function properly.
So ... there's no simple way to rebrand Inkscape at this time. If there's much interest in rebranding/customizing Inkscape, perhaps someone will put together a "customization kit" for Inkscape to make this easier, as has been done for certain web browsers. (would you be interested in starting such an effort?)