I recently upgraded to Drupal 7.17 and found that the contact form would generate the following error message:
Notice: Undefined index: copy in contact_site_form_submit() (line 155 of /var/www/html/modules/contact/contact.pages.inc).
I believe this is down to Drupal setting PHP error reporting to E_ALL and hence causing PHP notices to be shown, while the contact module generates the above PHP notice when it tests for the presence of the 'copy' field.
Not sure what the correct solution is. I turned off PHP notices. Perhaps it's better to have the contact module check the user ID before looking for the 'copy' field.