Looks like Google has announced that they will be providing hosting for the following 5 (most popular) AJAX Libraries:
in the form of their AJAX Libraries API project, described as:
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.
So the actual prototype.js file is loaded all 10 times and included in the page-load time. If the URL for that JS file was the same for every site, in this case, using the Google AJAX Libraries API, then that prototype.js file would be loaded once, and reused with all the other 9 pages.
Naturally this improvement in site load time only occurs is most of the sites you visit are using the AJAX Libraries API to host their AJAX libraries from, otherwise downloading all the AJAX libraries from the individual site will still be occurring for you.
And interesting step in a direction… this does give Google an awful lot of usage data from different apps around the world calling into their API, effectively reporting technology/site/pageview statistics indirectly.