Cognitive Daily has a piece on one of the latest studies of the effects of computer and videogaming on the lives of those who play these games, one that attempts to answer the question of whether massively multiplayer online roleplaying games, such as World of Warcraft, are actually addictive. It's worth reading if the topic is of concern to you. Spoiler: the study was unable to answer the question per se, but the results are mildly disturbing, in that it did show a greater tendency on the part of mmorpg players to drop less important activities, like sleep, school, and social interaction, in favor of the game.
A few thoughts on methodology and interpretation: As is pointed out in comments at Cognitive Daily, the "control" games, which included the coin-op arcade game Gauntlet, were poor controls indeed. I can't imagine playing Gauntlet for twenty minutes, much less going back to it again and again. A more sophisticated, even good, single player game like Half Life or Civilization would have proven a better control.
Second, some people are simply addictive personalities, and while the study was randomized, it doesn't answer the question of whether these games are more compelling to this sort of person than other habits which could become distractions, such as blogging, forum posting, online chat, or boozing it up in a bar. Non-gaming activities with a possibly addictive component might have provided a better control, but it would be scientifically irresponsible to encourage alcohol n00bs to get plastered.
Still, those who've played these games or are curious about psychology may wish to read the story for themselves, as it's an interesting take on a relatively new social phenomenon.