Free money is always good.
I have a link back here anyways, do I win?

Use of a cookie, would this not be hard work. Tracking a domain, then working out who owns it. Cookies can be rejected.
Customer ID's, I for one remove anything to do with affiliation and go directly to the source, so we might not actually gain anything from people signing up.
Going directly to the signup page would drive some people away too.
This will all be thought out nearer the time ...
Before you re-quote it, I know you said it.

There is already a referral field in our profiles, and when signing up, does this get used? That could indicate as to whether it's worth doing or not.
When joining this program do we just tick a box? I'll be happy to anyway, will only need to modify my current link back.