Take a look at this http://elementdesignllc.com/2011/11/contact-form-7-get-parameter-from-url-into-form-plugin/
Its plugin addon for contact form 7, it will grab any URL PARAMETER you have like http://www.YOURSITE.com/?aff_id=JOE
So if you have incoming traffic leads that are filling out your contact form 7, you will use the info as shown in the plugin I just sent
It will grab the aff_id=JOE part from the url
http://elementdesignllc.com/2011/11/contact-form-7-get-parameter-from-url-into-form-plugin/
I use it and it works as long as the AFF ID is shown in the url upon submission of the contact form 7.