Are ESI requests limited to originating backend?

[Lars Kellogg-Stedman]

Will <esi:include> in Varnish only fetch content from the backend that
served the document containing the <esi:include> directive?  Or can
<esi:include> be used to include arbitrary content?  That is, is it
possible to do something like this:

  <esi:include src="http://www.google.com" />

...and if not, is this by design?  Results so far indicate that ESI
requests always go over the backend that sourced the containing
document, but I wanted to verify this behavior.  Thanks!

-- Lars