I coded up a web-based IDE
two years ago for my senior project at California State University Fresno and presented it to a group of entrepreneurs at the Central Valley Business Incubator
. It was a student project, but it incorporated SVN on the backend and did some slick syntax highlighting in real time on the frontend. A user could login anywhere, code for a bit, commit changes and logoff.
They smiled and nodded, but it was evident that they had no idea what an IDE was let alone how a web-based IDE would enhance the development process.
It has been technically possible for years, but now with Mozilla's Tamarin, and Google's V8, more complex functionality can be added (webtellisense?).