ok...are you responsible for your client's content? Have you got something in your Terms and Conditions to say you're NOT?
If not, why not?
I understand as a responible host it's our job to police our servers as best we can, but the burden of proof doesn't rest with you. If you get a complaint, suspend the site and investigate. Till then, carry on regardless!
My advice. Not necessarily the best advice. Usual disclaimers apply.
