Saving as PDF and Cairo PNG does not take the custom background color into account: white text on a default white background is not rendered visible…
Inkscape 0.48.2 for Mac OS X saves the drawing as expected to PDF (and Cairo PNG) if a rectangle of the size of the page (or larger) is stacked at the bottom of all objects (try yourself with attached modified file).
Saving as PDF and Cairo PNG does not take the custom background color into account: white text on a default white background is not rendered visible…
Inkscape 0.48.2 for Mac OS X saves the drawing as expected to PDF (and Cairo PNG) if a rectangle of the size of the page (or larger) is stacked at the bottom of all objects (try yourself with attached modified file).