fobbman, Mr Isla and I battled over this a few years ago. I would go to night classes and he would babysit by putting our then 18 month old in his lap while he played Doom, Duke Nukem, Quake, etc. She is now a fanatic and will one day probably be a champion game player.
I'm not sure how I feel about it, but it was one of those things that I just had to let go. I'd rather her see violence on the PC than see Mom and Dad in mortal combat... it isn't a battle I could have won, so I decided to do what I could to balance things out instead. So now, she's a budding computer geek AND takes piano lessons!