Support for AJAX/AHAH form workflows

Relevant code snippet from captcha_validate() function:

  $captcha_info = $form_state['captcha_info'];

  // Get CAPTCHA session from CAPTCHA info
  // TODO: is this correct in all cases: see comment and code in previous revisions?
  $csid = $captcha_info['captcha_sid'];

  $solution = db_result(db_query('SELECT solution FROM {captcha_sessions} WHERE csid = %d', $csid));

I have a following scenario:
1. drupal form with image captcha.
2. There is a page that displays the form
3. form submitted using ajax request and form validate and submit callbacks always call exit()
4. attempts to submit such a form result in invalid captcha message form the very first attempt
5. debugging the problem leads me to think that captcha management in general is inconsistent.