There's usually someone filling the position of leader, at least on an unofficial basis, in any of our games where the PCs are an independent group with no higher authority in charge of them. And sometimes even if there's not an actual in-character group or if there is an NPC authority that's supposed to be in charge, there'll still be a leader.
Most of the time it's a job you get out-of-game; you're assertive enough to keep everyone on track, you make the best and most convincing arguments for what we should do next and how we can best accomplish a goal, and it translates into your character being the person who comes up with the big plans. Being leader doesn't mean you're a dictator, usually; our leaders tend to get a bunch of options in there, but once the consensus is reached they're always the one who tells the GM what the party is doing next. As a GM, it's nice because you can tune out the generally bizarre things the players are thinking about trying when they're talking among themselves, and just wait for the leader to tell you what their final decision is.
Generally, being leader also makes you the face man for NPC interactions, though if some other character has much better social skills they'll take over that part of the job (and you'll probably end up prompting them).
Being the leader is the next stage of being the note monkey; when you're the note monkey, you're keeping track of all the plotlines and bits of information the GM is throwing at you, but you're not trying to convince anyone about what they should be doing next. Once you get confident enough to start announcing what you think everyone should work on next, generally you've got their trust and you've assumed an unofficial leadership position. If that's what you want, anyway; you have to actually take the lead before anyone's going to think of you as the leader, so you could just as easily stay the note monkey and never have to worry about the rest of it.
What I think is interesting is that in our group, the dynamic of who is leader ends up having nearly nothing at all to do with the characters in the game. The leader is the player everyone trusts to make the right calls and be on top of the situation, and even if the PCs are generated with an eye towards making someone else the offical leader in-game, the de facto leader will always be that player's character. And there are some players who can make a PC that qualifies as a leader, can take notes and stay focused and make suggestions all the time, but the rest of the group still won't think of them as or treat them like one of the leaders.
It's also funny to watch what happens when one of the players who usually takes a leadership role decides enough is enough and refuses to do it in the next game, because suddenly the whole party is running around in different directions, and a lot of people will still think of that particular player and his or her character as the leader. Even the GM will.
(But even though it's often disruptive in that particular game, the fact is that the players who get recognized as leaders need to step away from that role every now and again, both because they get burned out on it and because it encourages someone else to take a stab at leadership.)
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i don't know if it's strictly necessary, but obviously it's strongly encouraged
ryan
Generally, it happens in game and in character - usually its the player who chooses to play a charismatic character and plays that character well. At some point the other players start to look towards that person for guidance. People who tend to be good leaders tend to play charismatic characters, though not always diplomatic ones. So I think that many leaders are a facemen but not all facemen are leaders. 
