Autocomplete textfield widget?
Try http://timonweb.com/how-create-ajax-autocomplete-textfield-drupal-7

ALSO: 
http://qandeelaslam.com/blogs/cck/converting-texfield-autocomplete-using-drupals-form-api
http://www.elevatedthird.com/blog/enable-ajax-autocomplete-drupal-7-webform-textfield

(Looking for a module,  now.)