How to I retrieve default checkbox values using Form API?
after you save it, it saves the value of the checkbox as #value (not #default_value).

When defining the field settings: 
(pulled from D7 cck_table.module)

  $form['css_class'] = array(
    '#type' =&gt; 'textfield',
    '#title' =&gt; t('CSS Class'),
    '#description' =&gt; t('Specify a class to be added to the table element.'),
    **'#default_value' =&gt; isset($field['css_class']) ? $field['css_class'] : '',**
  );


Revised:

More likely you would want to do this:

If you want to grab the default values, you can call `field_info_field($field_name


  Returns data about an individual field, given a field name.