As you likely know we here at port80 are very concerned about improving Web site performance. All the time we are considering new techniques for code optimization to be used in w3compiler or improvements to our server side products that address the mantra - deliver less, less often. However, recently some in the group have been questioning the mantra and occassionally sending more than is necessary by utilizing the idea of prefetching.
The basic sense of prefetching is to use the idle time the user has looking at a page to fetch the objects they are likely going to need in a moment. The prefetched items are put in a local disk cache and then if the user looks at the next choice, the page pops right up. Of course pre-seeding the cache is a dangerous game because it is also possible the user may not make the choice you expect. In this case, your bad guess doesn't necessairly cost the user anything (unless they pay by bandwidth use) since they wouldn't have noticed the background download; however, your site and server did more work and consumed bandwidth that never really gets used. Numerous research projects have attempt to automatically determine what to pre-seed a cache with by looking at a site's stats log dynamically and adding prefetch information, but unfortunately there will always be users who don't fit the normal browse pattern so you are bound to download stuff you don't need to in order to get the usability gain of improved perceived site speed.
Yet rather than get too worried about implementation details now take a look at prefetching. Probably the easiest way to do this is using Mozilla - see Link Prefetching FAQ We'd be interested in hearing any thoughts customers have on prefetching technology as we have been toying with the idea at p80 off and on for some time.