Drupal - How to get Form API #states selected value onchange?

Drupal #states is nothing more than the page containing JS data, and states.js reading them and adding necessary JS events to hide/show/etc fields. At the moment, there is no #states based method to get the selected value.

Note that it's perfectly OK to add #states to elements that are passed through Ajax. The beauty is that, whenever something containing #states is rendered, Drupal pipes these JS data along with the Ajax response. Since behaviours are run everytime after an Ajax response, states.js can update its visibility rules even for Ajax-changed elements.

I think your solution would be to use #ajax to do this. Your [#ajax]['callback'] will contain just-selected values in $form_state['input'], so you can use them.

Thanks @AyeshK and @Beebee for pointing me in the right direction. Here is how I finally got hold of the selected value and displayed it via an ['#ajax']['callback'] I attached to the node_reference select list. From there on it's quite simple to modify the markup to more advanced needs like displaying a preview of a node or a field.

In the end you might want to add a second wrapper with the same ID to hold the default value when editing an existing node.

/**
 * Implements hook_form_alter().
 */
function MY_MODULE_form_alter(&$form, &$form_state, $form_id) {

    if ($form_id == 'MY_FORM') {

        // add an Ajax callback to the select list and define a wrapper the action should happen in
        $form['field_node_reference'][LANGUAGE_NONE]['#ajax']['callback'] = '_MY_MODULE_ajax_callback';
        $form['field_node_reference'][LANGUAGE_NONE]['#ajax']['wrapper'] = 'MY-ID';

        // create a wrapper
        $form['MY_WRAPPER'] = array(
            '#type' => 'markup',
            '#prefix' => '<div id="MY-ID">',
            '#suffix' => '</div>',
            '#weight' => 3,
            );

        // do some magic
        if (isset($form_state['values']['field_node_reference'][LANGUAGE_NONE][0]['nid']) && $form_state['values']['field_node_reference'][LANGUAGE_NONE][0]['nid'] != '') {

            $nid = $form_state['values']['field_node_reference'][LANGUAGE_NONE][0]['nid'];
            $form['MY_WRAPPER']['#markup'] = t('You selected node number: @nid', array('@nid' => $nid));
        }
    }
}

function _MY_MODULE_ajax_callback(&$form, &$form_state) {
    // return the wrapper
    return $form['MY_WRAPPER'];
}