If we see good usage, we can work with browser vendors to automatically ship these libraries. Then, if they see the URLs that we use, they could auto load the libraries, even special JIT'd ones, from their local system. Thus, no network hit at all!
A great idea, but as far as I know not one that any browser has currently taken up. When Google announced Chrome in September I wondered if they might be the first browser to support such a feature. Today I finally got around to testing the latest Chrome using Wireshark and discovered that it doesn't yet do this, at least not for the latest jQuery release.