One thing... Mobile Safari didn't use to cache anything over 15KB uncompressed. http://www.phpied.com/iphone-caching/

(No idea about more current versions of iOS or browser-caching on other mobiles OSs.)

It's a reasonable point. Given the vast array of jQuery versioning across just the top thousand web sites, it's very plausible a user will not have each one of those cached, and then a lot of sites vary on what cdn (if not local) they're using.

I would be willing to bet that among just the top 1,000 sites, a user would be required to have 25 to 30 copies of jQuery cached to cover all the sites.