I suspect there are valid grounds for the desensatized argument in certain groups of people. It's certainly something that needs a psycholoical study purely aimed at the 'gamer' group, maybe using a non-gaming section of society as a base example.
I personal am certainly not desensatized by the legion of violent games I've played; Shooters, GTA and it's clones, action games, RPG-fighting, and in a way even Pokemon. There is certainly a certain satisfaction from doing well in these games, getting a high number of kills and in some small way ruining someone elses gameing session. And I know I get quite frustrated when a fight/whatever doesn't go my way.
With offline games like GTA etc, there is defenite satisfaction gained from killing random npc's at will, often in extremely violent ways; but usually this is because it either goes against the rules of the game you're playing (killing marines in Halo 1), or because they're so evidently not real, and the game is evidently not real, that it feels ok. You're shooting at pixels, not people. I also find a good COD session highly cathartic. It relieves any stress I've been building up for a while, and there's a real sense of achievment for climbing the score-boards and beating people who are (in ranking terms) better than you.
I still can't really watch gore-horror, or horror films in general without being horrified, and scenes of brutality in the more mainstream media also have the same effect. It's only going to be a tiny minority that play a violent game, and then go away and commit violent acts. Unfortunatly, it's those few people that the media tends to focus on. Kinda like whenever there's a school shooting in America and they find the shooter listened to heavy metal or Marilyn Manson.
By the look of the other replies, being able to cope with/enjoy/cause violence in a fantasy setting with ease makes it more shocking that someone would actually do that in reality.
