Looks reasonably good. Given that Arc also contains a server, it might be possible to do this. Basically it probably end up handling a cvs-like repository server-side, possibly automatically update-commit whenever a user does a change to his or her copy.
Currently, applications in the Arc server are writeable only by their owners. It may be possible to hack into prompt.arc to support applications being made wikied.
Shouldn't be too big. Basically you could simply set up a part of prompt.arc to use a revisioning repository tool, and simply leave the working copy of the apps in each user's arc/app/ directory. When an wikiable app is edited, you can do a "cvs update" - "cvs commit" or "git pull" - "git add" - "git commit" or whatever your backend revisioning tool is. If a conflict occurs, send it back for editing, otherwise we succeed in the edit and return to prompt.
Basically you just wrap around the revisioning tool, which is effectively what a wiki with edit history does anyway.
