According to http://www.michaelgeist.ca/content/view/6134/135/, IP addresses issued by American Registry for Internet Numbers (the US regional IP issuing agency) is under the SOPA's jurisdiction. The problem is that they give out the IPs for US, Canada, and a bunch of the Carribean.
ARIN assignments aren't enforced by any kind of automated means. If ARIN tells some Canadian company, "Sorry, SOPA says we have to kill your assignment", and the Canadian company says "Thanks, but we'll keep using 123.89.12.0/24 anyway", it's all up to the canadian company's routing peers whether they want to enforce ARIN's edict.
If these peers choose to ignore ARIN's edict, it raises a number of troubling questions, mind you - so we don't really want to go that route if we don't have to. But the point is, Canadian ISPs could very well take a stand against the US using ARIN to enforce SOPA overseas.
