kollel_wife: trust me every filter can be circumvented. the biggest problem with every filter is finding that line between too lenient and self defeating. that line is never perfect and is therefore always exploitable.
the only really impenetrable filter would be to route all of your traffic through a proxy and filter it through a whitelist, which is usually self defeating unless you dont mind manually editing a whitelist every time you need a site that isnt on there.
when i say self defeating i mean that the filter becomes so crippling that you get rid of it. a perfect example would be google.com. when you access google.com, youre really accessing around 6 IP addresses at once, because elements on the page contact different IP addresses and grab information from them. so gogole.com might not work as intended because those Ip addresses would not be included in your whitelist. another example would be gmail or yahoo mail which all draw data from a whole host of IP addresses and external web addresses when constructing the page you know as your email inbox, and all of those would be blocked by a whitelist.
i have a whitelist running where i work and trust me its a pain in the behind to maintain.