Would love to see how this evolves over time and what kind of issues are raised by the project.
Technological issues are significant (accessibility; usability; reliability; security; etc.) and, as Jim points out, can make the project a nonstarter. However, I am impressed by what I've seen of PBWorks so far.
The other big issue, of course, is cultural. A wiki is an "enabling technology", but having a perfect tool available doesn't necessarily mean that it will be used. A wiki-culture requires a high level of altruism (with people putting in significant amounts of work which rarely delivers immediate personal returns, but does benefit the "whole" in the long term).
If there is a mismatch between an organisation's culture e.g. a culture which is highly hierarchical and/or individualistic, then, at best, a wiki can serve as an "underground" tool to undermine the hierarchical and individualistic culture. And those individuals supporting this underground tool will need to be ready for quite a lot of antagonism and obstruction from those that are quite comfortable with the status quo. To give an example, it is quite possible that issues will be raised in a wiki which will openly question decisions which are being made by people at the top of the hierarchy. It takes managers that are already open to a more egalitarian form of organisation who will use this questioning positively. Others may feel very threatened.
Then, there is the more mundane task of facilitating the organisation of information within the wiki. Too much analysis without enough synthesis will result in a Babel Tower of seemingly useless information. Integrating information into a coherent whole is a thankless task, unless this is recognised, and supported, officially. Of course, Neill is the perfect person for spearheading this role, or, at the very least, building up the capacity within the organisation for others to take on this role

Here's a nice quote for you:
if specialised knowledge acts as bright torches probing around in the dark cave, then systems thinking provides the overall site map showing the extent and general features of the cave itself