I get the feeling that the various "online IDE" projects may be quickly superseded. If anyone could pull that off, it's Github - just getting Github integration working is hit-and-miss with most of the others. If it could include some live preview functionality for HTML/CSS/JS for mockups and Markdown for writing, it would quickly become vastly useful.

Does anyone know what text editor they're using? I assume it's CodeMirror, but I could be wrong.