When our party was three or four players, the GM said he'd run a session as long as there were two or three, respectively. We went to a fifth and as long as there's three players, the DM runs the session. I've since gone ahead and implemented the same rule with the group I GM. I think it's better for everyone. People who want to play still get to play, the people who can't make it don't feel bad because their absence cancels the game/don't feel obligated to play if they don't want to.