If you're worried about Google's reliability, you could always test for typeof(jQuery) !== 'undefined' before assuming it had loaded. Then, you can inject a script element referencing a copy of jQuery on your server if it hadn't.
In practice, I would think the chances of Google's entire CDN going down are the very least of your worries though.
That's quite an interesting idea, but I'd be worried about it running too soon, the browser doesn't give you a "gave up on downloading something" event.
Google goes down more than you think - it's not usually that google itself is down, but rather that DNS didn't look them up, or some anoying routing issue.
In practice, I would think the chances of Google's entire CDN going down are the very least of your worries though.