Good idea Hans.
As you propably know Mike Churchward has "adding a Wiki" fourth or so on his list of things to do to Moodle. http://moodle.org/mod/forum/discuss.php?d=3868
http://moodle.org/mod/forum/discuss.php?d=3909 where he defines a wiki. And here is a semi-integrated wiki but not the one that he was thinking of integrating.
But since Mike Churchward is very busy, there does not seem to have been much progress. A universally re-editable forum would be a quicker way of reaching this worthy goal, and would be a useful add on. So, please make it! It really sounds quite realisable doesn't it.
All it would really need is for the last post on the forum to be the default text in the reply box and, as is already almost the case in the reply window, all but the last post to be displayed only as links.
My guess is that Martin would be inclined to aim for more kosher, wiki-propper module as opposed to a "stop-gap solution" in the general release (even if there were time), but as you say, the forum is almost there, and of course perfectly integrated. So perhaps it could end up entirely kosher. And there do seem to be difficulties in ingetrating external open sources projects into Moodle.
(Off topic but..) This puzzles me a little... with all the wonderful open source software in the world, it surprises me that there is not more of it being stuck together, in a sort of meta-open-source, "open-sources format"!? Perhaps someone will invent some international standards for say, web deployable software and their data-bases, so that people can make things that fit together. Or that the database of phpnuke, or postnuke, or Xoops or Moodle will become such a standard. As it stands there is are many great wiki/forum/chat/quiz/calendar etc software being reinvented at various places all over the world. United we stand?
Resuing the forum module is certainly one way avoiding reinvention.
Good idea Hans.