When you configure a form label to be hidden via the webform component UI, the label tag is hidden. However, no "title" attribute exists on the form field itself making it difficult for users using screen readers to determine what the field is actually for.
The WCAG 2.0 guidelines at http://www.w3.org/TR/2010/NOTE-WCAG20-TECHS-20101014/H65.html recommends the use of the
"title" attribute to label form controls when the visual design cannot accommodate the label.
Attached patch fixes this issue for Drupal 6.
I'll provide a patch for Drupal 7 version later today hopefully.