I recently added a pull request to Html5 Boilerplate to remove the use of the Google CDN for loading jQuery. Little did I know that it would stir up some great debate about whether using a CDN, specifically the Google CDN, was the best practice or if concatenating and minifying your code into a single file is better.
The pull request was based on 2 things, 1 was a conversation Karen Ford (my co-worker) and I had with Alex Sexton at the jQueryTO conference in Toronto. The second was a very data-driven blog post Alex referenced by Steve Webster posted in late 2011 about Caching and the Google AJAX Libraries. In that post, Steve points out that there is generally very little performance benefit from loading jQuery from the CDN due to the amount of fragmentation that exists in what versions of jQuery are being used in the wild.