@mmottola pretty much nailed it on the head, but I'll try to add.
There definitely has to be a meeting (or two or three) a couple days prior to the Rumble. We met on Thursday to figure out our idea and a rough feature set. "Who does what" was basically decided by what everybody wanted to work on according to their skills or preference or lunar position. It would help to have two meetings, because one meeting is essentially brainstorming ideas of apps.
The rules are fairly simple - you can do sketches, ideas, stories, wires, etc. No screen ready assets, no code, no libraries with app functionality. (ie.
require "rails-rumble"; RailsRumble.run; # DONE)
Limited to 4 members per team - that includes the designer, the UX guy, whatever.
I'd likely be down with doing a Ruby or Rails specific DevTricks talk. We have an opportunity for an OpenHack or perhaps an evening earlier for everybody interested and who wants a crash course in Ruby/Rails.