This originally answered a supplemental question about the wisdom of downloading jQuery versus accessing it via a CDN, which is no longer present...
To answer the thing about Google. I have moved over to accessing JQuery and most other of these sorts of libraries via the corresponding CDN in my sites.
As more people do this means that it's more likely to be cached on user's machines, so my vote goes for good idea.
In the five years since I first offered this, it has become common wisdom.