It's not just games, of course. Yahoo has been talking about Inbox 2.0 for quite a while now; the idea is to put your email inbox in the center of your online social experience. Developers have been able to create applications for the Mail Application Platform for about a month now, but it's too early to tell whether the entire concept is gaining much traction among users (or winning over new users from Gmail, Hotmail and other competitors).
As Steve Rubel points out, it's not really clear how you're supposed to play these games with people from your address book; the games, which include the popular Mafia Wars and Mytopia, seem to have just been slapped onto Yahoo Mail without much real connection to it. Still, I'm glad to see Yahoo trying out something new and being one step ahead of Gmail and its Gmail Labs (which, admittedly, include the game Snakey, but it's not a multiplayer game).
Interested in more Yahoo Mail resources? Check these out: