Many studies have shown a correlation between violent pretend play and violent behavior. But what someone said above is key: real violence is very different from pretend violence, and much more harmful. Being beaten or living with a violent person is much more likely to lead one to violence than playing DOOM for hours on end (and yes, I know I dated myself there). What this brings up is that violence in the news is much worse than violence in James Bond. If I may briefly summarize the most excellent movie Bowling for Columbine (don't take my word for it, go see it!):
-the US is a country with lots of guns, and lots of people killed by guns. Americans are always scared of violence.
-Canada is a country with lots of guns, and very few people killed by guns. Canadians in major cities often don't lock their doors.

What is the difference? Some of it may well come from history (slavery, Civil War). But the most glaring symptom right now is the news: Canadian newscasts are relatively bland summaries of what's going on; they inform. US newscasts are crowded with images of disasters, death, and mayhem (even when there was no war); they provoke emotions, mainly fear and anger.

So back to violent video games: they are not good. But they are much less bad than violent reality in the news, which is itself less bad than getting beaten up. Perspective is important.