How can I use conditional logic to not display a  in page.tpl.php based on node id?
If you're trying to hide the left sidebar when viewing node 51, this is not the right way to do it. Edit the block visibility settings for the blocks in the left sidebar so they are hidden when the path is 'node/51'. Then in your template you can do this:

&lt;?php
  if ($left) {
    print '&lt;div id="left"&gt;' . $left . '&lt;/div&gt;';
  }
?&gt;


This will hide the entire div#left when there are no blocks in the left sidebar.