I read an article a few months ago (NY Times?) about service-included restaurants, which focused on a place in San Diego. They mentioned that the staff will literally refuse any extra tips that are offered above the service charge. If people offer to add an 'extra' tip on to, the staff says they can't accept the money for themselves, but would be happy to take the money and donate it to charity. The article said the majority of these offers are refused (by people who were perfectly willing to give the money away to the waitperson)! A very interesting look into the psychology of tipping.

In response to your query, if a US-based restaurant says service is included, I'd have zero qualms about leaving no extra tip.