try this
the code below sets the value of the hidden field, to the value of a custom field of your choice.
this tricks wordpress into saving the hidden field as the title ??
<div id=”titlewrap”>
<input type=”hidden” name=”post_title” size=”30″ tabindex=”1″ value=”” id=”title” autocomplete=”off” />
</div>
<script type=”text/javascript”>
$( ‘#customfieldofchoice’ ).blur( function () {
$( ‘#title’ ).val($( ‘#customfieldofchoice’ ).val());
});
</script>