Greg Burch and I have been working on a new framework for multiuser applications with the Flash Communication Server.
We have put up a pre-alpha version of a chat using the framework which you can view and use here.
The page includes info on how to get involved as well as how to submit bugs and suggestions.
Finally, this is pre-alpha and it by far not bug free or feature complete. In fact, the basic architecture on the server still needs a lot of work.