The results, although discouraging, are rather obvious. Violent movies and television have been known to desensitize people to actual violence, and now we know video games to be no different. What must be differentiated, however, is one's desensitization to violence and one's ability to cause violence. This study in no way correlates to the causality of violence.
Conducted among 257 college students, the researchers made each participant play one of eight randomly assigned video games (four violent, four non-violent) and then had them watch a 10-minute videotape of real incidents of violence. Heart rate and galvanic skin response measurements were taken before and after the game playing and throughout the viewing of actual violence.
For the record, the violent games were Carmageddon, Duke Nukem, Mortal Kombat or Future Cop; the non-violent games were Glider Pro, 3D Pinball, 3D Munch Man and Tetra Madness.
The full report can be found here (.pdf file).