There is a Quora topic for this at
http://www.quora.com/Conversational-Agents-Chatbots
Wikipedia is now referring to this as an “Automated online assistant”
http://en.wikipedia.org/wiki/Automated_online_assistant
The software may be referred to as a “Dialog system” (or Dialogue system)
http://en.wikipedia.org/wiki/Dialog_system
See in particular “Toolkits and architectures”
http://en.wikipedia.org/wiki/Dialog_system#Toolkits_and_architectures