"This bonus is calculated from a bunch of research we did on hundreds of thousands of game results to figure out how much of an advantage being in a team is. We do some behind the scenes adjustments as well for stuff like beginners paired with pros, etc."

Some opaque, out of context statistical data: http://www.leagueoflegends.com/board/showthread.php?t=16705

Personally, the matchmaking seems to me to get better and better the more players pick up the game. I still see unbalanced games, but I can't remember the last legitimate curb stomp, where one team was simply outclassed...