Of course adding this autotag is as easy as making an autotag plugin, such as the
youtube plugin posted to the forums last month.
In drshakagee's post just search and replace "youtube" with "topic".
Rename the variable $whichtube to $whichtopic.
Change this line that starts "$retval = ..." to
Text Formatted Code
$retval = "<a href="{$_CONF['site_url']}/index.php?topic=$whichtopic">$whichtopic</a>";
And that should basically do it (assuming I didn't make any typoes above).
So when is someone going to make a proper autotag plugin with an admin screen where you can create replacement strings for arbitrary autotags?