Off the top of my head, you can keep THE SAME list and just use a different optin code that has ad_tracking on it-BUT this requires setting up another optin page...
The problem with one list is that you will have people from different traffic sources on the same list and so you won't know WHICH TRAFFIC SOURCE best converts to sales - or am I wrong ..
This may not matter for SEO but for paid traffic where you are paying different prices per subscriber, it sure make sense to know which traffic source is converting to final sales, don't you think?
To summarize and add a bit, how would you go about tracking:
-which traffic source best converts to optins
-which traffic source best converts to sales
-which ad best to converts to optins
-which ad best converts to sales