After 3 consecutive sessions, a missing player should be booted. If they do come back, they'll be behind on the plot, (probably) behind on the XP, and acting foolishly on their way to certain doom due to any combination of those two factors. Remove their character from the game and when their life has settled down to the point where they can re-join, then have 'em bring in a new character that they know will be kicked out for 2 consecutive missed sessions.
Missing players can frequently be mitigated somewhat by having the character given over either to the DM or the other players. It's not the same, but it's better than missing a session.
If you don't have enough players to play for 4 sessions in a row, then you're pushing your luck on being able to continue the campaign -- you have to be prepared and plan ahead to have the down-time so that your journals and quest notes and the like are all prepped up and you don't end the previous session on an abject cliffhanger (like in round 4 of the second major combat of the day). Offhand, I'd say that three "oops" missed sessions in a row is probably going to doom the campaign.
That's my experience...
::Kaze (had a player that was making out with his fiancee upstairs during game time, so they played without him. And when his bard showed up for the final fight with only 2/3rds the levels of the people who'd been there getting XP the whole time, the bard died. Very quickly. Imagine that. So we've changed how we handle missing players since then...)