Most news outlets are reporting that the riots in Greece were over the shooting of a teenage boy. While that's tragic, when I first heard that, I was waiting to hear the "other" reason as well. Well this article may not spell it out, but I think it at least indicates that there's more to it than just the shooting:
http://theirishbulletin.blogspot.com/2008/12/what-are-greek-riots-really...
I also think we can safely expect to see more rioting around the world as we get deeper into this recession.