You quit………you lose.

Check out page 35 of the February 2005 issue of EGM (the one with the above cover.) In addition to the nice story on Halo 2, they have an H2 Multiplayer FAQ section. The 4th FAQ is

Q: "Does it count as a loss if I quit out of a game early."
A: "Yes. You automatically get last place."

Judging from all the folks I have seen quit out of games...I don't think many people know that. People may want to spread the word.