> You can do this already with Ajax (Asynchronous JavaScript and XML)
> but it involves a small amount of coding to fetch the bits of HTML
> from the server.

The HTML specification discussed here doesn't mandate, nor even define,
the support of scripting in all user agents, and the W3C's own agent,
Amaya, doesn't support it.  Security considerations mean it could never
be mandated, and offlining of documents mean it is not a general solution.

It does define the object element that will, in conforming browsers,
provide much of this capability, although I'm not sure that the semantics
of links in objects are well defined.  Validating XML parsers can also
use entities, which are the construct specifically designed for this,
although I believe there is also an Xinclude proposal.

My own solution to this problem would be to take HTML back to the its
roots and not to try and use it as a page description language, but,
rather, have browsers make appropriate use of link elements to display
associated navigation, etc.  However the cat is out of the bag and 
designers would not be happy with this transfer of control to the user.