COPPA (the Children's Online Privacy Protection Act) is a big deal here in the US. Compliace with the Act is required for websites that are directed at children. Whether or not your site is directed at children is loosely defined.
Ideally, you would want to be able to enable/disable COPPA-compliance in the user registration process and provide a way to configure key elements like where parents should send/fax authorizations.
I know that phpBB has a COPPA element, but I haven't really looked deeply at how it is provided. I've already encountered at least one potential client who would need COPPA support and would love to see it included in the future.
Just my two cents.
James