$form['cid']['#weight'] = 0.003;
$form['subject']['#weight'] = 0.004;

[subject] => Array
        (
            [#type] => textfield
            [#title] => Subject
            [#maxlength] => 255
            [#required] => 1
            [#post] => Array
                (
                )

            [#programmed] =>
            [#tree] =>
            [#parents] => Array
                (
                    [0] => subject
                )

            [#array_parents] => Array
                (
                    [0] => subject
                )

            [#weight] => 0.004
            [#processed] => 1
            [#description] =>
            [#attributes] => Array
                (
                )

            [#input] => 1
            [#size] => 60
            [#autocomplete_path] =>
            [#process] => Array
                (
                    [0] => form_expand_ahah
                )

            [#name] => subject
            [#id] => edit-subject
            [#value] =>
            [#defaults_loaded] => 1
            [#sorted] => 1
        )

    [cid] => Array
        (
            [#type] => select
            [#title] => Category
            [#default_value] => 1
            [#options] => Array
                (
                    [1] => General Enquiries
                    [2] => Support
                )

            [#required] => 1
            [#post] => Array
                (
                )

            [#programmed] =>
            [#tree] =>
            [#parents] => Array
                (
                    [0] => cid
                )

            [#array_parents] => Array
                (
                    [0] => cid
                )

            [#weight] => 0.003
            [#processed] => 1
            [#description] =>
            [#attributes] => Array
                (
                )

            [#input] => 1
            [#size] => 0
            [#multiple] =>
            [#process] => Array
                (
                    [0] => form_expand_ahah
                )

            [#name] => cid
            [#id] => edit-cid
            [#value] => 1
            [#defaults_loaded] => 1
            [#sorted] => 1
        )

function mymodule_form_alter(&$form, &$form_state, $form_id) {
  $form['#pre_render'][] = 'mymodule_form_alter_weight';
}

function mymodule_form_alter_weight($elements) {
  $elements['cid']['#weight'] = 0.003;
  $elements['subject']['#weight'] = 0.004;

  return $elements;
}

function mymodule_content_extra_fields() {
  $extras['mymodule'] = array( // Name of field
    'label' => t('Mymodule'),
    'description' => t('Mymodule field'),
    'weight' => 10, // The default weight, can be overriden on Manage Fields
  );

  return $extras;
}