We have squid 3 installed at our Linux gateway server. We have enabled caching. Can it cache objects from https sites ?
You would need to use the SSL bump feature http://wiki.squid-cache.org/Features/SslBump - though it makes no specific mention of caching (where it does mention adaption), so I don't know if it will actually work even then.
Of course there may be serious privacy issues associated with caching https traffic.