Show Colorbox or other Modal Window on page load
I figured it out, it's pretty easy. See below:

function popupmessage_init(){       
    $colorbox_js = 'jQuery(document).ready(function(){ jQuery.colorbox({html: "&lt;h1&gt;Your Message Here&lt;/h1&gt;"}); });';
    drupal_add_js($colorbox_js, 'inline');      
}