[emilv]

As long as they don't tamper with the data I think an HTTP cache is perfectly OK. HTTP has loads of built-in mechanism for that kind of caching. It saves bandwidth upstream, not least for website owners, and may make your web browsing speed faster if the proxy is good.

Tampering with the data, however, is not OK at all. In the U.S. I believe it may make the ISP exempt from for example the safe harbor clauses in the DMCA.

[ceejayoz]

Some of the big content providers like Netflix are even reportedly making caching deals for their content. Both the content provider and the ISP get better performance and less exterior bandwidth.