My latest battle with Drupal was overriding a select box that was a node reference. Intuitively I tried to simply update $form['#options'] with my array of custom data. No deal. After experimenting with about five ways to implement #after_build on both the form and the field, I realized that the type of a widget is something like 'nodereference_select' instead of just select. Switching the type back to 'select' allowed me to make the change. Wow. Lost a day to that one.

 function mymodule_form_alter(&$form, $form_state, $form_id) {        
   if($form_id == 'mycontenttype_node_form') {              
     // setup array            
     $data = array();                  

     // build your custom data           
     // i.e. write some code              

     // alter form           
     $form['field_myfield']['#type'] = 'select';           
     $form['field_myfield']['#options'] = $data;          
  }
}