Set up Drupal to use alternative nodes for different incoming urls?
You could use the mod_rewrite features in your site's .htaccess to redirect before Drupal even gets started. This would increase performance vs. using JavaScript.

Implementation details:

https://stackoverflow.com/questions/5151385/redirect-on-the-basis-of-referer-http-referer-htaccess

More about Drupal's .htaccess:

http://drupal.org/getting-started/clean-urls

Contents of Drupal's .htaccess:

http://drupalcode.org/project/drupal.git/blob/HEAD:/.htaccess