It seems to me that unless the likelihood of a cache miss is fairly small you need to balance the probability of a cache hit against the expense of an extra HTTP call, as opposed to bundling the JQuery libraries directly with your custom JS with some JavaScript minimization trickery (and two HTTP calls if you're using both jquery and jquery-ui).

I have no doubt that the likelihood of a cache hit here is growing, but I wonder what the likelihood of an actual hit is? These data show that 4.7% of the top 1000 Alexa sites use some version of JQuery. What you'd need to consider is the likelihood that your visitor has (a) visited one of the those 47, (b) that is using the same version of JQuery as you are, and (c) has done it recently enough that the (relatively large) files are still locally cached. I suspect that for most sites that works out to much more than 4.7%, but is it more than 50%? If not aren't half of your users getting a slower response as a result?

(Moreover, and I don't know if or how this effects the JQuery CDN, but doesn't it seem like many sites drag because of delays in loading the Google Analytics JavaScript files? Wouldn't this pose an even greater problem if you're using Google to serve JQuery, since your UI depends upon it?)