For what it's worth - I needed to keep NextGEN Gallery active, but when it was active Contact Form 7's Javascripts did not load in the footer.
So, I manually added the links to the scripts in my theme footer:
<script type='text/javascript' src='/wp-content/plugins/contact-form-7/includes/js/jquery.form.min.js?ver=3.40.0-2013.08.13'></script>
<script type='text/javascript'>
/* <![CDATA[ */
var _wpcf7 = {"loaderUrl":"\/wp-content\/plugins\/contact-form-7\/images\/ajax-loader.gif","sending":"Sending ..."};
/* ]]> */
</script>
<script type='text/javascript' src='/wp-content/plugins/contact-form-7/includes/js/scripts.js?ver=3.5.2'></script>
I'm sure that's not the best way to do it, but it seems to be the only way I can have both plugins active at the same time, and once I did it this way, the on_sent_ok function started working.