Also looking for this feature.
A quick hack which works for me is to modify simple-tags.client.php for the version of wordpress in use (in my case 2.5). This only works on PHP5+
Find function autoLinkTags (~line 247) and a bit further down find $token = preg_replace($match, $substitute, $token); // only PHP 5 supports calling preg_replace with 5 arguments
Add an extra parameter to preg_replace at the end with the value of 1 so it becomes:
$token = preg_replace($match, $substitute, $token, 1); // only PHP 5 supports calling preg_replace with 5 arguments
This will match the first occurance of a word/tag in a paragraph. Not ideal, but its better than every tag.
I'll leave it for the plugin author to make the necessary changes to the actual plugin ;) Hopefully in the next version.