To create an ‘id’ element is straightforward:

xml.id 'my-id-here'

and then to put that tag in the ‘foo’ namespace you need:

xml.foo :id, 'my-id-here'

To create children of that element (say xhtml content in an atom feed) you need:

xml.content :type => 'xhtml' do 
  xml.xhtml :div do
    xml.p entry.excerpt
  end
end

And the final piece of the puzzle, and the end to that particular bout of frustration is the text! method. To use my exact example:

xml.georss :point, :featuretypetag => 'wifi' do 
  xml.text! entry.latitude + ' ' + entry.longitude
end