"Public cacheing" has little merit, as efficiency of
cacheing decreases when covered population grows
beyond some threshold (i.e when diversity of requests
overwhelms the cacheing capacity -- process better
known as "thrashing").
On the other side, small populations do not produce
aggregatable demand patterns.
I.e. it looks like that ISP-provided cache servers would
be optimal. Conviniently, that allows to sidestep the problem
of finding cache servers. When you never talk to other provider's
cacheing servers you don't need to locate them.
There are two ways to do distribute load: "supply side"
caches (like secondary DNS servers) and "demand side"
caches (like cacheing proxy servers).
The supply-side cacheing has a fundamental problem is that
you cannot tell which supply-side server is best. So much
is clear from the DNS saga.
--vadim