[mistercow]

This is very cool, and would definitely be useful whenever you don't need to know much about layout or overall page state before your code starts running. Here's some feedback:

I worry about the idea of waiting until late-appearing elements show up if you're selecting on the id. It's tricky to use that feature correctly because ids are globally unique. To handle ids sanely in flexible code, you have to automatically generate them (using, say underscore's uniqueId). Then you pass them around so your code is all talking about the same element. Yuck. At that point, we might as well pass around callbacks.

So you could solve most of that if you let waitUntilExists work for class selectors too. Then the user would just refer to it by the context, passing "itself" as the last parameter.

Which brings me to my last point of feedback: why "itself"? Seems to me that it would make more sense to just have the element be the default context if no last parameter is provided (or if it's undefined).