Looking at a couple of tools I found on the web CGIProxy and PHProxy it seems to me that content based blocking by companies becomes a bit pointless, as you can put one of these scripts on a home PC on a DSL/cable modem connection and bypass anything which blocks based on URL, unless you use a "everything not explicitly allowed is denied" setup, which is kind of a hard sell in most companies.
Additionally, if you access these over a SSL connection, any proxies or content checkers won't see anything apart from the original URL so content scanning wouldn't work either...
Just goes to show, open one port on a firewall and be prepared for the fact that almost any content can come through....
