Copy value of field into the value of a different field How can I copy the value of one field into another during a node edit?  

I'm trying to take the value of a text field and have it populate the title field.  I'm thinking I can use the Ajax framework and $form_state, but I haven't quite got it yet.  Or, is there a way to do this on submission?  The Title field is hidden so Ajax isn't required.

In case this helps, the scenario is this: user edits a node, fills in one text field and that value automatically populates the Title field in addition to the current field.
@Volker's tip gave me everything I needed and it's pretty simple as it turns out.

if (!empty($form_state['values']['some_other_field'])){
  $form['title']['#value'] = $form_state['values']['some_other_field'];
}


Set that in a hook_form_FORM_ID_alter and it works great.  The part I didn't have was the #value.