Well, there are many ajax terminals already. Making an ajax terminal launch a secured (chroot, no-call-to-outside-stuff, restricted, etc) telnet to 127.0.0.1 or a textmode mud client might be easier.
Moving forward and taking advantage of the power of the web browser is the key reason for wanting to make an AJAX MUD Client. What I have seen are simply re-creations of existing command line terminals and their limited capabilities or seek to tunnel through TCP/IP to telnet by going through the webserver. (So really the webpage is not connecting to the MUD, but the server that the page is from is connecting.)
MUDs that would want to use the web browser as the client, allowing improvements on existing muds from simple things such as allowing clicking on an exit to go that direction or click on an object to look at it or pick it up. More advanced and new features such as allowing profile pics for characters, so looking at a character will show the portrait. Using the browser as a client will allow the beloved text input interface to be preserved while introducing more modern capabilities that are universally supported. MUDs will have to code specifically for these capabilities and will also have to retool communications to work over TCP/IP.
That is the direction I was thinking a project like this should take. I am interested in working on the client, but I don't have plans on developing a MUD server. That is part of why I'm here, to see if any MUD Admins would be interested in adapting their mud, making my effort worth while.