Dr. Lorenza Colzato at the Leiden University has headed up a study on working memory among gamers and non-gamers. The results were published Psychological Research, and the team put together the video above to spread the word. Perhaps unsurprisingly, the group of gamers completed the working memory task more quickly and more accurately than the group of non-gamers.
Colzato attributes the result to the idea that first-person shooters "require players to develop a flexible mindset to rapidly react and monitor fast moving visual and auditory stimuli." Working memory is defined as "the part of short-term memory that is concerned with immediate conscious perceptual and linguistic processing," so it's important to note that this study shows nothing about long-term memory; playing Call of Duty is still probably not the best way to study for an exam.
Video Games: Good or Bad for Your Memory? [Science Daily]