<foo>
  ...some elements...
</foo>

<bar xmlns:xsi="http://www.w3.org/2000/10/XMLSchema-instance"
     xsi:noNamespaceSchemaLocation="[somewhere]">
  ...some elements...
</bar>

(yes, the transformation from foo to bar is pretty trivial at
the moment, but it's expected to get nastier later).

Of course the first thing I tried (since this is the first time
I've used XSL) is exactly what they tell you not to do in
section 7.1.3 of the XSLT spec --

<xsl:transform xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
               version="1.0">
  <xsl:template match="foo">
    <xsl:element name="bar">
    <xsl:attribute name="xmlns:xsi">
      http://www.w3.org/2000/10/XMLSchema-instance
    </xsl:attribute>
    <xsl:attribute name="xsi:noNamespaceSchemaLocation">
      [somewhere]
    </xsl:attribute>
    <xsl:apply-templates/>
  </xsl:template>
  ...some templates...
</xsl:transform>

-- you know, the part where it says "Thus, while it is not an error
to do [what I just did], it will not result in a namespace declaration
being output."

but I confess that (1) I don't understand it, (2), it looks like
overkill, and (3), I have to think that anything involving

has got to be The Wrong Thing for something as simple as what I'm
trying to do. The couple of FAQs below it don't look like what I
want either, or else the explanations aren't clear enough.