In a Villanova University study (PDF), 167 undergraduates played games that were either violent or non-violent, and were then asked to choose an action for a fictitious character in a story. The gamers who had just played violent games like Mortal Kombat or Doom 3 "produced significantly more aggressive responses" than those who'd played inoffensive fare like Tetris Worlds.
Seems pretty conclusive, doesn't it? Professor Patrick Markey, who conducted the study, doesn't think so. "Their personality made a big difference. People who are extremely angry tend to be much more affected by violent videogames than people who are not angry and of course the opposite is true that people who are not angry are virtually unaffected by violent videogames. So it's both the person and, in essence, the situation."
Markey, himself a gamer, cautioned against sensationalism from the mainstream media, as well as knee-jerk defenses from -- ahem -- game journalists. "I like the rating system so legislatively I hope nothing's done. ... [but] getting out the information that videogames can have an effect if you're [an angry] person is extremely important." For still more on Patrick Markey's research, you can check out his appearance on the Cheap Ass Gamer CAGcast.