Index: modules/field/modules/options/options.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/field/modules/options/options.module,v
retrieving revision 1.9
diff -u -p -r1.9 options.module
--- modules/field/modules/options/options.module	27 Aug 2009 00:33:51 -0000	1.9
+++ modules/field/modules/options/options.module	5 Sep 2009 20:18:09 -0000
@@ -200,6 +200,12 @@ function options_select_elements_process
     '#options' => $options,
     '#default_value' => isset($element['#value'][$field_key]) ? $element['#value'][$field_key] : NULL,
   );
+  
+  // If field is required and has one option, select that option automatically.
+  if ($field['cardinality'] == 1 && $element[$field_key]['#required']) {
+    $keys = array_keys($options);
+    $element[$field_key]['#default_value'] = $keys[0];
+  }
 
   // Set #element_validate in a way that it will not wipe out other
   // validation functions already set by other modules.