It would wake up a lot of people very fast. You've just hit on one of BTC's biggest weak points, if not THE biggest. There's been a lot of new mining set up in Canada and the U.S. in recent times though (to go with what already exists in places like Iceland, etc.), so I don't think it would be the end by any stretch of the imagination, but, could it freeze up for a week or two? Take all that hash power off line and the diff adjustment would happen very slowly.