Interesting, since when? I'm curious about how it's turned out in practise. For web services I mean. An for anyone hosting a message board or comment section.
The UK law is actually a good implementation if you put child 'safety' as your number one priority, with any other considerations as, in practise, moot.
Unfortunately I think free civil discourse between adults, privacy, etc. are just as important as child safety which makes the current law a bit crap.
This is similar to the video game and MasterCard/VISA issue - you can buy games that promote sexual violence and incest. Nothing stops children downloading them for free, or using their under-18s debit card from purchasing the non-free versions. In this instance it was private companies leveraging their freedom of association rather than an all encompassing law from a sovereign state, but the intent is the same.
As a collective society we do really need to come to grips with what it is that we want. Allowing kids to freely access gang torture/execution videos and playing pro-rape entertainment should probably be tackled. I'm not sure I agree with the implementations though.