I really enjoy the list that Fluent Simplicity started in May with a pretty comprehensive list of companies with an active Twitter presence, but I have yet to see real business results reported out.
For a long time I have been interested in how people are using Twitter for business, crisis and other useful applications. I have heard many anecdotes but little in the way of hard business results. I am in search of these case studies. If you know of any, drop me a line, or leave a comment in this post.
In the meanwhile, I will continue to collect them in my delicious account called Twitter Uses, and will probably institute a new one called Twitter Results (UPDATE: I started adding case studies). If you don't have your own blog, or if it wouldn't be suitable to put it there if you are a big brand, I would even be willing to write up a case study for you and publish it on Communication Overtones.
This week, ABC carried a nice piece about how some companies are having success using Twitter as a customer service tool, and from experience I can tell you that it can be powerful. But I am still uncertain if anyone has quantified the effect of it on their company.
Let me know if you hear of anything out there that is interesting along those lines.