The biggest challenge of a communal startup lies in maintaining participation.
Given the diverse talent in Seattle, how can a project developer balance
the sub-projects so that as many people as possible get to contribute?
Two ways (others may exist) become apparent: 1) pre-organize the sub-projects by getting as many decisions out of the way before the event, or 2) allow people to pitch their ideas and then everyone can align themself with the idea they like.
The first way implies that the organizers reveal the decisions ahead of time and then people choose to show up, so participation remains more constant because the people have selected the project, instead of showing up and via consensus, having a project chosen for them.
The second option also allows people to choose their project. The risk lies in not finding any project "worth their time". Their only recourse lies in grouping with other uninterested people and forming a coffee klatsch answer party (similar to Saturday House's "Intelligence On Tap") where people from understaffed other ideas can ask for ideas how to do something.
Staging a Six Hour Startup or any startup gathering benefits from tons of preparation. Using intelligent defaults for architecture (e.g. providing Subversion, Trac, and access to a VPS) will ready the scene for people to start building immediately. Beyond preparing the scene, all organizers have to do is get out of the way of people and their ideas.