I currently work at Instructables.com, and we wrote a GPL’ed library called JSLoad that does a similar thing. It’s been working beautifully on IE, FF, and Safari for almost a year now, serving about a million page views a day.

One difference between using and JSLoad seems to be that the dependencies are managed centrally by a JSLoad singleton; we work with many existing libraries and scripts, and adding dependency code to files we didn’t create seemed like a maintenance and upgrading headache.

JSLoad was also written with flexibility in mind. Dependencies come in the form of “tags”, which allows for all kinds dependency structures that are independent of the js files themselves. This was very useful since I was handed a good amount of legacy code when I started; JSLoad allowed me to encapsulate the code according to initial guesses and refactor for efficiency and maintainability over time.

Here’s the link JSLoad.

@MorganRoderick: There will only be one extra delay per library that hasn’t been loaded, and that delay only occurs once. This is the same delay that occurs when the user hits the page; it is only lazily loaded. Ultimately, it’s up to the developer and scenario to determine whether this is feasible UX or not.

And, it even adds more complexity to your code.

But, under the right conditions, I suppose it’s great.