Variable substitution not working?

I just chased down the same problem. Since you're using the dev version, this ought to work for the value of the body field:

string("$title")

Not exactly intuitive to me, but then I'm not really that familiar with xpath either. Perhaps it's obvious to those in the know.

As a clarification, $variable substitution, substitutes the value from another XPath query into another XPath query.

For instance:

<feed>
  <entry>
    <title>kitten</title>
    <link kitten="http://example.org/"/>
  </entry>
</feed>

To get the value of the albino attribute , you could do:
context: //entry
title: title
link: link/@$title

link would becom:
link: link/@kitten

This needs further documentation.

You can use them that way, but they won't be the same. The value that the XPath query "title" returns will be used as the XPath query for body.