How to use xmlns declarations with XPath in Nokogiri

I'm using Nokogiri::XML to parse responses from Amazon SimpleDB. The response is something like:

<SelectResponse xmlns="http://sdb.amazonaws.com/doc/2007-11-07/">
  <SelectResult>
    <Item>
      <Attribute><Name>Foo</Name><Value>42</Value></Attribute>
      <Attribute><Name>Bar</Name><Value>XYZ</Value></Attribute>
    </Item>
  </SelectResult>
</SelectResponse>

If I just hand the response straight over to Nokogiri, all XPath queries (e.g. doc/"//Item/Attribute[Name='Foo']/Value") return an empty array. But if I remove the xmlns attribute from the SelectResponse tag, it works perfectly.

Is there some extra thing I need to do to account for the namespace declaration? This workaround feels horribly like a hack.