Google works directly with the key stake holders for each library effort and accept the latest stable versions as they are released. Once we host a release of a given library, we are committed to hosting that release indefinitely.
There's also more information at Speed up access to your favorite frameworks via the AJAX Libraries API.
To play devil's avocado briefly, the question is - can we trust Google enough to build functionality around them? It might be a moot point if you're already using their APIs, and you could always use the libraries directly, but it's worth considering.