BuddyPress.org

There is a WPMU plugin, which asks the user to agree to terms of service on first login (not during registration): http://wpmu.org/terms/

I have added a Terms of Service page to my site. Then I added this statement/question (required) to the Basic Profile: “I agree to the Network Guidelines”. There are two radio buttons (yes and no) and “Network Guidelines” is linked to the page which details them.