Process the tableselect #multiple = TRUE submitted values.
function _form_test_tableselect_multiple_true_form_submit($form, &$form_state) {
$selected = $form_state['values']['tableselect'];
foreach ($selected as $key => $value) {
drupal_set_message(t('Submitted: @key = @value', array(
'@key' => $key,
'@value' => $value,
)));
}
}