This paper on Microsoft’s web site shows the inner workings of the new TrueSkill Ranking System. It includes notes on how the matchmaking works, how the leaderboards are maintained, etc.
If you’re into videogames, and like stuff like
|exp(-(μA-μB)2)/(2*c2)) * √d|
|c2 = 2β2 + σ2winner + σ2loser|
|d = 2β2/c2|
then this is the article for you!