An unstructured wiki with revision history is not the ideal application for creating a co-authored document. A relatively small team might fare better using a network-based word processor. The gap between wikis and word processors will widen as features found in today's desktop word processors -- control over document format and layout, outlining, footnotes, etc., -- are added to the network programs.
But, further structure is needed if we are to create massively co-authored documents, documents created by very large teams. We can see a start toward that end at PublicMarkup, a platform for public discussion of legislation. The following example was taken from the discussion of section 101 of the Emergency Economic Stabilization Act of 2008. As you see, recent comments are displayed next to the text of the bill, and one can see all of the comments and also add their own.
One can imagine extending this system to allow people to vote on comments. Users could also register and maintain profiles on the site, enabling them to contact others interested in the same portion of the bill -- to form communities of common interest.
Intel and portable PC manufacturer ASUS have incorporated such social networking features in a project where they are asking the public to design a "dream" laptop, net access portable, and gaming machine or to submit a cool design idea. While a wiki is totally unstructured, they have added some structure to the user input. Here is the form for a dream PC:
This is the form for an idea:
Finally, the user can submit drawings or links to videos along with their dreams.
Users can vote ideas up and down and add comments to them. Intel and ASUS hope these votes will help them focus on widely demanded ideas and the comments will help refine them.