Ironically, Eclipse doesn't want a browser-based Eclipse either.
http://www.eclipse.org/orion/
Their Orion project takes a very different approach of integrating services from all over the web and includes GitHub integration out of the box. It does have a server side part, but it also has a ton of client side code (that's all BSD-licensed!)
Their rendering is based on contentEditable, which means that it handles RTL text better than anything else and is likely to be accessible to disabled people. (For these reasons, we're working on including this editor in Firefox!)
As far as straight up programming editor functionality goes, Ace is hard to beat. It's got extensible keybindings (someone has built vi bindings, emacs is also possible). And don't let the DOM part throw you off... keep in mind that a canvas is DOM also – you could draw a canvas minimap if you wanted to.
And, as others state, if you really want to use canvas for rendering, you can pick up the Bespin code. The last version produced didn't have the server side and all of that stuff.
I think JS-based editors are the future, but I don't think brand new editing components are where the action is at: the action is in the stuff around the editing component.
(ObDisclaimer: My biases come from having worked on Bespin and Mozilla's other developer tools...)