I have vague memories of reading a post with a stronger conclusion — that CDN caching was basically a non-issue, it would so rarely work — but to my frustration I can't find that right now.
* an IP,
* a referrer from the first page on your site that included the script (no subsequent pages, because now the client has jQuery cached).
All in all, a pretty poor source of information compared to AdWords, Google Analytics, G+, hundreds of millions of Android users, and running the world's most popular search engine. At most they could crunch some browser stats or jQuery usage stats, but they already get that and more from their own services + crawling the web.
And even if this tiny amount of info were somehow a boon to Google, so what? It doesn't hurt you as a site owner.
tl;dr for lazy HN readers: "using Google's CDN to load jQuery isn't likely to benefit the majority of your first-time visitors." As of 2011, jQuery 1.4.2 was by far the most common version and even that was only loaded via googleapis.com on 2.7% of websites.
Note the difference in version Steve found vs my comment (due to query strings etc., which will of course destroy caching)