In particular, about how to share the traffic load properly when multi-homed. In a sense, since most of inbound content is probably porn, I suppose you could call it Breast Augmentation and not be far off. Anyway, two of our T3 customers practice it. Basically, in Europe and Australia, most ISPs get in 3-10 times as much data as they push out. When a national provider' s network is having a very bad day, aggressive caching is a way to get the users zippier access to the data.
More disk is necessary, but the key factor is a larger user population.
The more end users that are hitting on the cache, the greater likelihood that the cache will have a higher hit rate, because the chances are better in a larger user population that 2 people want to go to the same place on the Web. It's possible to set up hierarchies of caches using ICP, the Internet Cache Protocol, and to configure your cache to get the data requested users of other caches in the cache hierarchy, thus simulating a larger user population and making your cache more effective. A simple modification allows you to simulate the larger user population. Having cooperating caches work together as above is one way of prepopulating caches with content. Often, this prepopulation is done when bandwidth is cheaper or when the network is just less utilized.