I'm a huge fan of Remember The Milk and GNU Emacs. So much so, that I've given them both money over the years. And now I've (kind of) combined them. I just pushed slack-rtm to GitHub.
I've kind of avoided Emacs Lisp, the 1984 Subaru of Lisp. While I've done some minor hacking of my .emacs in the two years I've been using Emacs, I have not really done any serious programming in it. But with this, I'm hoping to remedy that a bit. It's been fine so far, even though the learning curve has been steep. I don't think I'll be writing a JavaScript interpreter anytime soon.
Right now, it's pretty simple. It just creates a buffer and shoves your tasks into it. I've only tested it on GNU Emacs 23.1.50.1. Ultimately, I'd like it to sync between an orgmode file and RTM, kind of like what Sacha Chua did with org-toodledo.
So, have at it. Feel free to fork it and send me pull requests. I'm hoping to release early (DONE!) and often.