> use <abbr>SQL</abbr> if you want you
> audio-browser to say ess-cue-ell, and use 
> <acronym>SQL</acronym> if you want
> it to say "Sequel"

Not quite.

This is a fantastic article:

   http://redish.net/content/papers/interactions.html

A couple researchers sat down with 16 blind people to see how they use
the Web.  Their results are very enlightening.  Here's one of the
things they found:

Unusual words, acronyms, and abbreviations confuse screen readers, and
screen readers often read them wrong, e.g.

   Word on the screen    	What the screen reader says

   homepage                      	hommapodge
   LiveHelp                      	livahelp
   MEDLINE                       	Medlynepalus
   FY                            	fie
   VA                            	va (like the Spanish for "go")


So, independent of whether you consider XML to be an abbreviation or an
acronym, do not depend on the tag alone:

    <acronym>XML</acronym>
or 
    <abbr>XML</abbr>


For accessibility, always use the title attribute:

    <acronym title="Extensible Markup Language">XML</acronym>
or
    <abbr title="Extensible Markup Language">XML</abbr>

/Roger