[development] Forms API

[Steve Ringwood]

I wonder if it's because you are missing a final call 
drupal_render($form) in your theme function?

Also a suggestion for simplifying your code, when you build the form 
change the loop with invoices to

  while ($invoice = db_fetch_object($invoices)) {
    $fieldname = 'invoice-' . $invoice->invoice;
    $form['invoice'][$invoice->invoice] = array(
      '#type' => 'checkbox',
      '#title' => $invoice->invoice,
      '#default_value' => 0,
      '#cp_data' => array(
        'invoice' => $invoice->invoice,
        'total_due' => $invoice->total_due,
        'date_due' => $invoice->date_due,
        'status' => $invoice->status
      )
    );
  }
  
$form['invoice']['#tree'] = TRUE;

Then instead of looping through the whole form/form values you can loop through invoice
and have the invoice for the key  getting rid of the need for string manipulation.
Some like

  
  foreach ($form['invoice] as $invoice => $element) {

or

  foreach ($form_values['invoice] as $name => $value) {

nevets