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.

/**
 * 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'];
}

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'];
}

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.

/**
 * 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'] != '') {
            $form['node_reference_preview_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'];
}

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.

/**
 * 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'];
}