Keep in mind that generally now, IMs are thought of as more 'disposable', and people write one or two sentence posts (as mentioned above) quite often. in informal discussions, the majority are pretty empty 'hahah' or 'lol' types of things.
additionally, people who are used to this culture of 'fast, short messages' in chat will be a little thrown off if you then ask people to post thoughtful and insightful messages via a chat medium. longer messages take time to read, and the constant automatic scrolling of text as messages come in will be really frustrating to participants.
Besides the advantage of easier readibility due to NOT having automatically scrolling text like in a chat system, an advantage to using a more "lagged" system or "asynchronous" system is the perception of more permanence to the messages. people will generally put more thought into their replies, and people used to the cultural difference between IM and forums won't be annoyed when a person sends a couple paragraphs in what most see as a disposable chat medium. forums "look" like websites, so the content in the forum looks/feels more permanent. EVERYONE can read it, even long after it's posted if the forum itself is kept visible. Even if it's disclosed early on that all forum messages will be deleted after the discussion, there's still the perception of permanence because it's on a solid "static" webpage.
so, technically yes, chat is a non-threaded forum, but there are differences in perception and the effect of those differences that could have a big impact on the quality of discussion by participants.
Just some stuff to think about.