Prof. Dmitri Williams, one of the leading academics in the gaming space, who was an expert witness at the U.S. Senate on the constitutionality of game banning laws, held a roundtable on game addiction at GDC. Williams referenced Carnegie Mellon University A. Fleming Seay's research on addiction in an MMO context.
The addiction, defined as "problematic use" by Seay, is when an entertainment product dominates and displaces other behavior,
causes conflict and not playing causes anxiety. Seay found that self regulation (self monitoring, self evaluation and self consequence) was difficult for certain people. This seems obvious, but like anything in academia, if somebody hasn't done the research it isn't legitimate. The other thing Seay found was people's affinity for a game caused their problematic use -- but the interesting part was that those with "problematic use" were those who didn't like the game they were playing. They would do the same actions over and over again, didn't find pleasure in their actions and got stuck in a rut.
Williams thinks now is a the time to start looking into the ideas of "gaming addiction" and "problematic use" before it gains traction. He says, "Coming up with a solution to that, is better than sitting in front of Congress in ten years."