Does your company need a wikipedia policy now ?
Untill now Wikipedia has not been in blacklists - even after the placement of some viruses it kept its white knight image. But with Wikiscanner that can all change now. Now anybody may look up whatever change was done from your network on wikipedia and you may become the laughing stock or the 'big manipulator' on the web.
Any user on your network will be identified with your natted IP address. This is the single point of identification. It will not say that the firm itself has done it but some employee. And when you go through the changes people are making, you sometimes ask yourself if their boss knows about this .....
What you should ban is that nobody in the firm should bring any changes or comment on anything on the web regarding your firm, its business or its news stories without having the agreement of the communications department. This is the only department where one can decide about the opportunity of changing or adding things to forums and wiki's just like wikipedia.
It shows also that communications departments of firms should employ webwatch software and have some people (freelance or not) to keep an eye on things. The development of a 'rapid response strategy' could be handy in case you need it - and you won't have the time to decide about such things.