I've implemented something similar in Loggly's shell based UI. One of the things we needed to implement was dynamic content - images and graphs alongside text content in the browser.
I was inspired by http://goosh.org/, which is a nice little command line interface to Google.
Shellinabox is interesting for me mainly because it looks web-like (in contrast to things like anyterm and ajaxterm) and presents normal pty device on server side (and thus can run arbitrary interactive unix programs). Also it is easiaer to deploy than anyterm and seems more reliable and faster than ajaxterm (and also it is more flexible because it can pass used URL to service that it wraps).