Thanks Nick!
That's just what I was looking for I opted for the jQuery method. I'm somewhat of a novice when it comes to jQuery / javascript so I'm not sure if I went about it the best way, but within the infinite scroll plugin settings there's an option box for a callback.
Since I'm bringing in a chunk of posts and I only want to insert the code once per chunk and not after ever post I used .after() to insert a div after the 'loading new posts' graphic. Then used .load() to insert my ad code into the div I just added.
Does this seem right? It's working I'm just curious if I went about the code the right way.
jQuery('#infscr-loading').after('<div class="isad"></div>');
jQuery('.isad').load("/wp-content/themes/base/includes/infinitead.php");
Thanks again,
Marty