Here's one of my favorite links on the subject of educational games and how they (should) work, which I've used to good effect as a youth librarian:

Games and Learning

The chess club at my regular branch has at least 25 people every week, and at my current branch I leave a chess set on one of the tables and often see people playing it. I try to incorporate gaming as often as possible; it's less about teaching than about fun, and as Dr. Bartle says the learning should be incidental. I've added Kerbal Space Program to the teen tech lab Macs, and the kids have learned (without realizing it) a little about aerodynamics. I also ran a Dungeons and Dragons campaign for three months, and got them doing art and reading fantasy novels out of it.