Perhaps interesting in that context is that Twitter seems to have started to track clicks for users of the web site. (Clicks go through a referral page so Twitter can count them).
I don't recall them announcing anything about what they want to do with it, but it seems like it could become part of a commercial offering.
Unfortunately it only works for users on the web site.
We have thought about putting in a beacon of some sort. An issue with that is that a beacon (like #zs8) is one of the first things to be removed from a RT if it's not a valid hashtag. A url, however, is rarely removed from a RT since it's pertinent to the message.
We're still thinking about how to track RTs from the timeline. The new retweet API will most likely help and make timeline campaigns more valuable.
An "opt out" page on our site is a good idea. One issue is we don't do an interstitial on which to display that link. Have to figure out a good way to make that visible. Thanks.