try going to program/include/rcube_template.php and find
$input_user = new html_inputfield(array('name' => '_user', 'id' => 'rcmloginuser', 'size' => 30, 'autocomplete' => 'off'));
and change it to:
$input_user = new html_inputfield(array('name' => '_user', 'id' => 'rcmloginuser', 'size' => 30, 'autocomplete' => 'on'));
the option to do this from the template was not in the 0.2a release but has been added since.