This module is now released on Drupal.org (http://drupal.org/project/referralsources). All issues in this thread should be resolved. Please file any bug reports or feature requests using the project issue tracker on Drupal.org.
The Referral Sources module allows you to ask your users "Where did you hear about us?" anywhere on your site, and track that data in a central location. It provides a admin interface for creating referral sources that users can select from a list, and a page where you can view statistics on referral source selections. Referral source submissions are stored in a single table, no matter where they came from, making it easy to track submissions and create reports based on that data.
The core Referral Sources module allows you to easily embed a referral source selection to the user registration page, or your own custom forms and modules.
The Referral Sources (Ubercart) module adds a checkout pane prompting users to specify a referral source, and a order pane so you can view referral sources selected on when viewing individual orders.
The Referral Sources (Webform) module adds support for embedding the referral sources selection in any webform.
The Referral Sources (Example) module demonstrates how you can add the referral source selection to your own forms and modules.
* The Referral Sources (Ubercart) module requires Ubercart.
* The Referral Sources (Webform) module requires the Webform module.
See included README.txt for full installation and configuration instructions.