SVG specifically allows foreign namespaces. The validator marks them as invalid.

The following example, taken from the SVG 1.1 2nd edition specification returns the error:

"Element piechart from namespace http://example.org/myapp not allowed as child of SVG element defs in this context."

<?xml version="1.0" standalone="yes"?>
<svg width="4in" height="3in" version="1.1"
     xmlns = 'http://www.w3.org/2000/svg'>
  <defs>
    <myapp:piechart xmlns:myapp="http://example.org/myapp"
                    title="Sales by Region">
      <myapp:pieslice label="Northern Region" value="1.23"/>
      <myapp:pieslice label="Eastern Region" value="2.53"/>
      <myapp:pieslice label="Southern Region" value="3.89"/>
      <myapp:pieslice label="Western Region" value="2.04"/>
      <!-- Other private data goes here -->
    </myapp:piechart>
  </defs>
  <desc>This chart includes private data in another namespace
  </desc>
  <!-- In here would be the actual SVG graphics elements which
       draw the pie chart -->
</svg>

See: http://www.w3.org/TR/SVG/extend.html