very good question ekim. Actually, I can imagine that you think that, chatbots.org would have, should have, the answer is to to this question. We are working on a report like this indeed. However, this will take a while. we might have this information available, mid or probably end 2010.
We currently only our directory tab and our company profile list:
http://www.chatbots.org/companies/
furthermore, I’ve estimated that the long-term potential of this industry is as large as the advertising industry today, which is US$600 billion( no typo). This presentation might help:
http://www.erwinvanlun.com/images/pres/Erwin_Van_Lun_-_Chatbots_as_a_Business.pdf
Hope this helps