I've been away from my desk this afternoon and just thought I'd reply to this whole thread.
I think the cynic in Mark is right. I can't really see us disconnecting a customer for running an open resolver. We'd be more inclined to try and help them fix the issue and then potentially block their port 53 traffic internationally. It's just the way we operate in general.
Our AUP doesn't forbid customers running servers at all and I don't think that should change at all.
I think the biggest scare tactic is the traffic overage. We've seen along the lines of 25GBytes in 10 mins when a botnet went "a bit wrong". Obviously not all that traffic made it to the customer involved...
Dave