Funny, sitting in the drive through today, my mind wandered to what it'd take to reimplement python.el in terms of an IPython back-end process. While I'm not terribly interested in notebook support per se, I'd love to be able to run OS X Emacs.app against multiple IPython "REPL servers" running on OS X, FreeBSD, and Windows. This project might be a good place to start.
Speaking of Windows, anyone out there have recent status on any efforts to port IPython to IronPython? IIRC removing CPython implementation dependencies was explicitly mentioned in the general design goals for the 0.1x refactoring, but I couldn't find anything beyond this when I looked into it a few months ago. I ask because it's actually a project I've seriously considered picking up myself if it's not nearing completion elsewhere.