My Two Coppers...
Ah...the gaming Mecca of GENCON...I have no idea if I can make GC '04, but something like this might make it happen

!
After reading through all the suggestions thus far, here are my humble suggestion(s):
(1) Set-up: Tables of 8, with DM, Asst. DM and 4-6 players, battlemat, egg timer, minis, pens, water and rags. Add-in 4-8 support staff to handle the action matrix, observe the action, etc. Have a "success thermometer" posted in a place easily seen by all (more on this later). Use pre-generated PCs (good mix, core rules only), allow 5-10 mins for character selection at the beginning of the game with everyone rolling a d20 and select in order from highest to lowest (allow a few minutes for horse-trading of PCs at each table). Scenario rules are published in advance for those that sign up (with an extra copy or 2 at each table).
(2)Scences: I think TB is right on with this, but here is how I suggest working it - in 2-3 stages...
Stage 1 - There is a big threat to the kingdom (pick your bad guys - orcs, gnolls, undead, etc). The duke (king, prince or whatever) has issued a call-to-arms to protect the kingdom and each group gets a specific mission (designed to be accomplished in a fairly short period of time) such as:
* Scout the enemy forces location (2-3 groups, one for each axis of advance)
* Diplomatic mission to convince a reluctant baron to provide additional forces (1 group)
* Protect a supply caravan moving to the city (2-3 groups, each protecting different materials)
* Hold a key pass to prevent a portion of the {insert enemy} army from linking up with the main force (1-2 groups)
* Retrieve a minor artifact from an insane wizard (1 group)
* Convince the thieve's guild to suspend thefts during the crisis (1 group)
The more groups that accomplish their mission, the more resources are available for
Stage 2 and/or Stage 3. If groups fail to accomplish their mission, then the "Success Matrix" is not advanced, making the future stages tougher. Use the "Success Thermometer" to keep a running record on how the groups are doing and to build tension.
Use an "If/Then" action matrix or flow chart to determine what happens in subsequent stage(s). The support staff keep the matrix updated with what is going on. PCs that are killed in
Stage 1 are dead, put their players can now join the enemy side, taking the part of key NPCs and assisting DMs with combat duties or simply observing. The surviving PCs rally together and then divided (either randomly or to build balanced groups) and given
Stage 2 Missions. This allows people to play with a variety of different ENworlders over the course of the adventure. Also, since some the players of the dead PCs will be assisting the DMs by taking the role of key NPCs, the remaining groups could be a bit larger (6-8).
Stage 2 Missions might be:
* Counter-attack to take a key bridge, allowing kingdom forces to attack an enemy flank (1 group)
* Protect an important summoning ritual (1 group)
* Poison the enemy's water and/or food supply (1 group)
* Destroy an enemy seige tower or battering ram (2-3 groups)
Additional casualties are likely, with dead PCs switching sides and taking enemy NPC roles. The climax of the event is
Stage 3. Depending on attrition in the original PC ranks, there could be enough former PCs playing NPCs to make it pretty much ENworlder vs. ENworlder with the DMs as referees. As in
Stage 2, the remaining PCs rally together and are reformed into new groups. The "Success Matrix" and "Success Thermometer" are advanced and the finale begins, with each successful mission providing more goodies and/or advantages for the final stage.
Stage 3 Missions might be:
* Hold the breach (1 group)
* Lead the counterattack (1 group)
* Attack the enemy command tent (1 group)
The overall success depends on how far the "Success Thermometer" is advanced. My recommendation would be to have the earlier stages count less than the later stages, so success or failure of the final missions determines everything.
A number of awards could be given for those that survive the entire run (I was thinking of a "best RPer" chit given at each stage by either the DM or table acclaimation - Stage 1 chits worth 1 pt, Stage 2 chits worth 2 points and Stage 3 chits worth 3 points). A "first-to-die" award, "best one-liner award" and anything else that can be thought up.
There is a time limit for each stage and groups that fail to meet the stage goal are considered to have failed...this should keep things moving along and prevent the "Hennet shops for a MW belt buckle" type things.
Anyway...my 2 coppers! I can't commit to being there...but I might make it for something like this!
~ Old One