For those that were curious
I'm very impressed by the willingness to help from the community here. Thanks a ton everyone!
Here's what went down:
I wrote my own adventure in Masterplanner (amazing tool!). My 8 players used Character Builder and sent me their sheets. There were quite a few conversations in Vent about decisions, rules, etc, before the game.
We tested our tools (ScreenMonkey on the player end - just a web page) and opened an Obisidian Portal page for the campaign, seeding the world with a map and some details about the local regions.
I ran a quick 10-30 minute diceless prelude with each player, sometimes in pairs (or one trio) to establish their characters background, and set up the RP environment for the first game.
I used Masterplanner's combat maps only locally, mainly so I could use it to track initiative and NPC hitpoints.
Sunday night, we started at 9 sharp and went to midnight sharp. By the end of the evening, everyone had a fairly good grasp of the basic mechanics, they had completed a skill challenge, a combat, and done at least 40 minutes of actual role-playing.
Screenmonkey seemed to buckle under more than 4 users, and there were complaints about it being very slow to update. After our first session, I get 5 or 6 players to stay behind and help me test Maptool with them. It worked very well, and we will be using it starting next week.
I still feel like 8 players is too many, but now I know it doesn't have to be the-sky-is-falling-and-we're-all-going-to-die too many.