For what it's worth:
I've done two multi-player scenarios at conventions:
1. Here: http://leadadventureforum.com/index.php?topic=84319.0
Four-player, 3x3 feet table. Modified Snatch and Grab: four minor plot points, major plot point in the center of the table. Major plot point cannot be attempted without getting one of the minor plot points. Each league had 4 characters: leader, sidekick, and 2 allies. Each league started within 6 inches of a corner. This game, including teaching the rules, took less than three hours. Players were two veteran adult wargamers, two teen-aged newbies. No problems, an unexpected twist at the end, and great fun had by all!

2. Just played last Saturday and will post an AAR here soon: Six-player, 4x4 feet table, Hidalgo Fire scenario. Four minor plot points, major plot point in center of table, players deploy one character at a time until all leagues are deployed; characters may not deploy within 6" of an enemy or a plot point; player with the initiative deploys first. Same type of leagues: leader, sidekick, 2 allies. All six players had D&D experience, but none had miniature wargaming experience: two adults and four teen-agers (2 male, 2 female....the winner was one of the ladies!). Started teaching rules at 9:00 am, started play at 9:30, finished at 1:30. Could have finished sooner, but the adult players(!) played very deliberately. As gamemaster, I did my best--in a positive and humorous way--to speed things along, but not always with success. No problems, great fun had by all!

It was a measure of the players' enjoyment that even though the winner had for all intents and purposes been determined early in turn 6, the other players insisted on playing to the very bitter end....there were many grudges and personal vendettas between the various characters to resolve!

I have all the available Fortune Card decks....in the six-player game I had to reshuffle in turn 5.
gweirda, I contemplated using the red herring cards, but I chickened out and didn't use them! I wish now that I had!
In both scenarios it was every man--and, er, woman--for himself...er, herself...uh, themselves?

Next time I think I'll try something different...each league paired with one other as uneasy allies, who may not fight each other but may play fortune cards and challenges on each other. Can't let them get too cozy.
Also, we used a protocol for the playing of challenges: the first player to the active players left, or clockwise direction, has first choice whether to play a challenge. If he declines, the next player to the left may play a challenge, and so on. And, of course, the players can talk it over--negotiate, threaten, or cajole--before a challenge is played! Got that idea from Dave in some blog or other.....
