Under one iteration of SOPA, the copyright holder could file a private action to have the infringing site shutdown. Alternately the copyright holder could file an action with the US DOJ to have the site shut down. In either scneario the first action is to shut down the site, then notify the site of infringement.
Since SOPA has not been enacted into law yet it’s really an open question. If you trust the supporters of the law you have absolutely nothing to worry about because magical ponies will ensure that the law only is enforced against evil nasty "foreign" web sites.
