several folks have sent replies to me personally which may be of general interest:
Hit rates are user population dependent. We have seen hit rates as high as 30% from a single T1 connected customer (with a population of less then 500 people).
so have we. but that was a computer club with its own POP and they were mostly sharing interesting (i.e., pornographic) URL's over an IRC back channel. we've got ~80 transparent caching units out in the field, and most of them are in "normal" modem pools where the users don't know each other and the communitities of interest have broader splay.
... in some special cases you really can predict what would best be in the cache beforehand. You then win from a speed standpoint (pre fetched data is served up faster).
yes, that's true -- in some special cases you can exercise prescience and if you can then the hit rate goes up by a lot as does the downstream data rate. however, to make this work you have to continuously feed the primary caches new prescient data (or prescient metadata by which prescient data can be fetched). interest factor is in other words dynamic and shortlived.