Information architecture is to writing, editing, and laying out pages as urban planning is to house-building. Collaborative information architecture is like letting individual house builders re-route traffic, decree zoning rules, bulldoze apartment building, drain lakes, open parks, erect bridges. It expands the role of the information creator from the limited range of text and media to the musical interplay of ideas.
Collaborative information architecture is one important way in which Wiki differs from a MessageBoard. On a message board, users may post messages, in series (usually), with optionally some stock images or (in some cases) images of their own provision. In a Wiki, contributors can move, fold, merge, branch or re-route others' contributions, and organize those contributions in different ways from different perspectives.
This is, perhaps, the eye-opening part of Wiki: not that anyone can edit a page, but that anyone can edit a site.
Information architecture is a difficult field in which to collaborate, however. Most people have an idiosyncratic intuitive cataloging method; to all of us, one or a few ways of organizing information just "makes sense", and others don't. The encoding of cataloging methods into rigid standards -- the LibraryOfCongressClassification? or the DeweyDecimalSystem?, for example -- can make it seem to some that there is a natural mode of classification and organization of information, with no brooking of dissent.
Differences of opinion on points of fact or presentation are relatively easy to identify and resolve; differences of opinion on where things go and how to get there can be a little more painful.
A list of some tasks associated with information architecture in Wiki.
In [A Manifesto for Collaborative Tools], Eugene Eric Kim (of [BlueOxen], which hosts PurpleWiki) discusses what he believes is a necessary approach to improving collaborative tools. Quote: "This essay is a manifesto about software for collaboration -- why the world's future depends on it, why the current crop of tools isn't good enough, and what programmers can and must do about it." Also published in the May 2004 issue of Dr. Dobb's Journal.