This is planned, but is not in fact in use yet. Update - I have learned that the Apache server , as of version 1.1, can also offer a cache proxy function.
Current browsers have good coverage for configuring proxy via DNS name, port, excluded domains etc.
Harvest added access control during development of version 1, and squid has this too. Essential in a situation like Glasgow HEP, where users may be registered to get access to documents on the basis of their calling domain, and the cache is also in that domain. The cache is therefore a potential trojan horse. However, this is embarassing since we have to refuse peer requests from other caches. Hensa schedule an if-modified check at reasonable intervals.