So from 1.1 to 1.2, validation changed too. Apparently 'alphaNumeric' validation is just that (26 letters, and 10 arabic numerals). The following worked for a required field, which could not be left blank, but would allow spaces and alphanums:
'fieldName' => array(
'rule' => array('custom', '/^[a-z0-9 ]*$/i'),
'required' => true,
'allowEmpty' => false )