There is still a round in Toronto for the 2008 season. The main reason for combining the series is to avoid crowning two champions for what is essentially one series anyway. For the fans who dont closely follow the sport it is somewhat confusing when they crown two champions in Vegas. This also ensures complete participation in every round from riders who are shooting for the championship. They may add an additional round outside of the U.S. for the 2009 season and beyond. Its likely the future international rounds would be in Canada or Mexico just to keep things simple for the race teams. The one thing the teams seem to dislike most about the international rounds is getting their equipment, bikes, and parts to the venue. That is the main reason why a lot of teams arent for rounds overseas and many teams have to put in a lot of effort to send riders to races like the Motocross des nations.
As far as the rules go, I am not sure how they plan on handling the differences between the FIM and AMA rulebooks for the SX series. It is likely that they will make one set of rules for the combined series but I havent heard anything about it yet.