UPDATE:
Okay, I figured out what I was doing wrong. As of right now, when logging into the forum, you are automatically logged into the wiki.
However, the login from the wiki doesn't exactly work right, as it still tries to tell my that my user doesn't exist. Also, once logged in to the wiki (after using the forums login page) the logout script doesn't seem to work either.
Either way, we went from not-working to mostly-working.
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OLD:
Dokuwiki has what looks to be a user-management extension class that lets the wiki not just use the punbb for authentication, but it also shares sessions.
I haven't gotten it quite working yet locally, but here's the relevant link:
http://www.dokuwiki.org/auth:punbb
If I can get this working, I'd lean heavily towards DokuWiki as the one to use.
PS: Please add this to the State of the Step page.