Isn't that the point of using Google's copy of jQuery for your sight, in lieu of hosting it yourself. You can share the cache with any other site that is pulling the jQuery lib from a trusted 3rd party. After looking at the link you posted, that code may be simpler with respect to size, but in terms of maintenance or having a novice manage it (without introducing cross platform issues), it seems like it may be a nightmare.
What would be nice, and Adobe has done this for flash, is to have a separate cache for trusted libraries. It would be nice if I could query to see if jQuery were available in a browser lib cache, and use it from there if available.