I'm curious about this point - specifically because some sites (I'm interested in the general category of sports betting/poker) seem to be blocked, while others with very similar content don't get blocked.

I'm sure there are a million and one ways in which these CAN work, but I'm interested in what the 'typical' (if there is one) method employed by big companies is. I assume it is some centralised solution, i.e. doesn't reside on the client machine

Is it url based? meta-tags? page content?

To take this forum as a notional example - would it be possible to have two 'skins' at different domains (domain aliases?), presumably residing on the same machine, accessing the same database, but using a different front end which maybe didn't have the ads and gambling-related keywords?
How would one do that - register a domain alias and point it at a subdomain of twoplustwo with a different forum front end?

I'd imagine a typical company of a given size uses a service which has already pre-screened a lot of web sites and provides their ratings, then the company can decide which categories of sites will be allowed and which won't. Depending on the service the list can be very dynamic or be rarely updated.

thanks kerowo - I suspected as much.

regarding the other questions about having a domain alias point to a subdomain - do you know if that is possible?

That I'm not sure about, or at what level the blocking takes place.

There are many ways to employ blocking. Most all firewalls have keyword/URL name based blocking options. Some firewalls have subscription services that block sites by categories. There are companies that provide alternate DNS services that block sites by category http://www.opendns.org is one.

By redirection of a sub domain I guess you mean you want to do something like workaround.mydomain.com to get you to 2p2? You could point it at the IP for the message boards, you could also edit your HOSTS file by adding 204.232.160.145 forumserver.*********.com then if your company is using OpenDNS or a service like that it might get you around it, but it may not be 100% reliable to get you there all the time. If your firewall is blocking based on keywords then it will catch keywords on the pages still.

thanks icyflops !

How do workplace internet filters usually work?

Badly.