This is a sample Drupal module that modifies an existing form field to display it as a multiple select without having to modify the handling code. As far as the form's "owner" is concerned, the user enters a comma-separated list of values.
The goal is, as you can probably tell from the $form_id, to turn a profile field into a multiple select of values from elsewhere. This is probably less than ideal, but I can't figure out a better way to achieve this.