Arrays should be formatted with a space separating each element (after the comma), and spaces around the => key association operator, if applicable:

$some_array = array('hello', 'world', 'foo' => 'bar');

Note that if the line declaring an array spans longer than 80 characters (often the case with form and menu declarations), each element should be broken into its own line, and indented one level:

$form['title'] = array(
  '#type' => 'textfield',
  '#title' => t('Title'),
  '#size' => 60,
  '#maxlength' => 128,
  '#description' => t('The title of your node.'),
);

Note the comma at the end of the last array element; This is not a typo! It helps prevent parsing errors if another element is placed at the end of the list later.