Comment 1 for bug 313122

The file that you link to should actually validate as it is valid xml. The problem is is that the validator does not seem to be namespace aware (or at least cannot coupe with multiple namespaces in svg documents see: http://www.w3schools.com/XML/xml_namespaces.asp for info on namespaces.

Inkscape svg's have an inkscape: namespace to help with editing the file in inkscape, as allowed by the xml specification. If you want to save files from inkscape without the extra namespaces, then you need to save as Plain SVG. This only contains the SVG namespaced elements, and validates according to the w3g validator.

Use: File -> Save AS -> Plain SVG

For more information see this FAQ:
http://wiki.inkscape.org/wiki/index.php/FAQ#Are_Inkscape.27s_SVG_documents_valid_SVG.3F