I see this impacting non-targeted tweet campaigns primarily. For those who build twitter profiles that zero in on particular demographics or use twitter to maintain an ongoing conversation with people looking for specific content, retention is not a problem. I run quite a few mainstream blog accounts on twitter (my main twitter account
http://twitter.com/websitewriters is not one of my blog accounts) and let me tell you--less is more. Sounds very Zen, right? I focus on a lower number of users for my own blogs' twitter pages but feed them the info they need. In terms of monetization success, I run Adsense on these blogs and we're looking at many $3 clicks, sometimes higher. Per click. Targeting is key. Also, since the person is following you and listening to you, as long as you satisfy their content preferences and you do it strategically (sending out updates that have high relevance but also high PPC value), your effort is rewarded.
Other marketers' results will vary of course. Variation results from niche (is your target market looking for info or are they impulse buyers?), how you find and target people to follow/recruit, how active you are in sending @ and if the @ messages also have relevance to people looking at your tweet feed or the tweet feed of the people responding to your @ I can't say it's a consistent cash cow right now but it is a source of income that can be a nice component in your overall multi-traffic/multi-revenue stream overall strategy.
Just my 2cents on the matter.