Hi Tom,
 I hacked together a preliminary solution that uses the booktabs and
threeparttable (for footnotes) packages,  based on the suggestion in
section 13.2 of http://orgmode.org/worg/org-tutorials/org-latex-export.html
It seems to do OK in a simple example, and I will test it some more. It
does not export to html.
Thanks for your help.

#+LATEX_CLASS: article
#+latex_header: \usepackage{booktabs,tabularx,threeparttable}

* noexport   :noexport:
#+tblname: test-table
   | L | R$^1$ | X$^2$ |
   |---+-------+-------|
   | A | B     | C     |
   | D | E     | F     |
   | I | J     | K     |


* do export
#+name: tabularx-export
#+begin_src latex :exports results :results latex :noweb yes
  \begin{table}[htb!]
  \centering
  \begin{threeparttable}[b]
  \footnotesize
  \caption{A table to test booktabs}
  \label{tab:test-table}
<<booktabs(table=test-table,align="lrX",env="tabularx",width="0.5\\textwidth")>>

  \begin{tablenotes}
    \item [1] A table note ...
    \item [2] A second table note ...
  \end{tablenotes}
\end{threeparttable}
\end{table}
#+end_src


On Fri, Dec 28, 2012 at 2:06 PM, Thomas S. Dye <tsd@tsdye.com> wrote:

> I think the :align attribute expects a string instead of a symbol, so
> :align "rr" is probably what you want.
>
> I don't know about the new html exporter yet, but I'd be surprised if it
> looks at LaTeX attributes.  You might want to experiment with alignment
> cookies in the table to see if they do what you want:
>
>
> http://orgmode.org/manual/Column-width-and-alignment.html#Column-width-and-alignment
>
> I haven't used threeparttable or ctable.  Org footnotes work inside
> tables, if you want the note to appear outside the table.
>
> Typesetting tables is hard.  Let the list know if you get Org to work
> with either of these packages.
>
> All the best,
> Tom
>
>