I found the following question on the salesforce.com customer community forum... I believe this is the issue you at hand... his question was never answered there.
We are using a wordpress-to-lead plugin on our new site, integrating the contact forms to create new leads in Salesforce. It's set up to feed 'Lead Source', but this only displays the name of the page the form is on within the site. How can we set up 'Lead Medium' field to collect the referring url for analytics?
Thank you!