Hi, been lurking here for a while. My first post. :)
You could just check if the $teaser variable returns anything and have it spit out anything inside the conditional statement.
<?php if($teaser) { ?>
<p>I'm a teaser.</p>
<?php } ?>
I'm doing something similar but it's set to a class then I can control it from my style sheet.
You can read about it here.
http://www.dvessel.com/node/87
It covers a few more things too.