I don't really see this as a replacement for an IDE, but it might be a very nice tool for collaborative code writing (e.g. during code sprints), for working when travelling and away from your main development machine, and for literate programming.
The nearest equivalent I can think of is Bespin, although I think that's just an online code editor, it doesn't let you run the code.
It might be nice to run a CodeNode instance at NeuralEnsemble, although it would probably be best on a separate server: I imagine you could quickly bring a server to its knees if you have many users at once, or large data sets. What do you think?