As posted by somebody else it's also worth using a local fallback. html5boilerplate has been doing it for jQuery for some time now.

This can be done with most libraries by identifying the libraries global variable. e.g. for Modernizr

<script>!window.Modernizr && document.write(unescape('%3Cscript src="/public/scripts/libs/modernizr.js"%3E%3C/script%3E'))</script>