This is just a fantastic idea. It's very simple. Use Google's extensive infrastructure to serve those common JavaScript libraries you love to use. I've recently started using jQuery so all I have to do is point the link to the URL provided by Google and that's 17kb per request my server won't have to turn out. Thanks, Google!
This comes from one of the best new sites out there "Janko At Warp Speed". This article shows us how to handle a few common message types within our applications. It's all CSS and very sharp!