Just posting a fix for anyone who might need it. :)
It took me quite a while to figure out why the category/product info templates weren't working for me. The default template worked perfectly, but nothing else did. After some debugging (which was made MUCH easier thanks to the included debugging options!) I finally found the culprit.
My host (Dreamhost) uses PHP-CGI, thus causing $_SERVER['SCRIPT_NAME'] to return "php.cgi" instead of the actual script name. On every page, STS was looking for (and couldn't find) a "php.cgi.html" template instead of the actual template name - "product_info.php.html" for example.
Easy fix! Just make this change (instead of turning off PHP-CGI if that's an option) and you're good to go.
File: catalog/includes/sts_display_output.php
Line: 47 or so
Change:
$scriptname = getenv('SCRIPT_NAME');
to
$scriptname = getenv('SCRIPT_URL');
That should do it. :)