This is seriously worth checking out. For it to be really useful for geeks, the keyboard shortcut interface has to be masterful (ie: worth learning). Or, like others are suggesting, at least replicate significant portions of vim. Doing 95% of the main ex/vim commands would be super easy. Emacs in the browser has probably been done by somebody somewhere, too, but... ugh. A big chunk of vim would be more than enough for a good app. Plus vim is more hip these days anyway.

If there's source available, I'd love to run a hosted version of this for myself. Unfortunately, I couldn't use this for work-related tasks, because any meaningful task list for me is going to liberally reference details of my company's IP.

Edit: Idea: Defaulting to HTML5 offline storage if available would be much better, with remote storage required to use sharing features. Don't take your user's data if you don't have to, right?