its no different than wikipedia linking to their own pages.
It's actually quite different, as I see it. On Wikipedia, editors work together toward creating a single encyclopedic page over which (after publishing) they can still exert control, but of which they ultimately have no ownership.
Forums are vastly different in the sense that members contribute individual posts in which they express their personal thoughts, feelings and opinions. Because you sign your posts with a nickname, you own your words -- you are not, as is the case on Wikipedia, anonymously or invisibly contributing text to a collective endeavour. Scroll down on WebmasterWorld and you'll read: "Member comments are owned by the poster."
The trouble with converting parts of users' posts to 'relevant' links is that it implies that the member who has originally posted the message, has also inserted the link. After all, noone but yourself and the post's author will suspect that you have begun to link certain words and phrases to certain pages after the post was created.
To answer your question: yes, I believe the technique is rather unethical, because you would be changing the original intent of users' messages.
Automating the process of converting phrases to links is especially risky. To illustrate: imagine one of your members has written an emotional post about the recent death of his beloved cat. A few weeks after the thread has last been active, he/she returns to it and finds that one or several occurrences of the word "cat" have been exploited, as it were, to link to a page about cats in general and thus wholly unrelated to his/her cat. A poor illustration, perhaps, and the effects likely differ from one forum to another, but I assume you recognize my point of concern.
I think londrum's suggestion [of making it so that this only affects older posts] is good.
Remember that just because posts are old, that doesn't mean they are no longer read. As you mentioned, "forums can be great generators of search traffic, most often for long tail terms," and much of that long-tail traffic will likely be directed at older posts, so that they will be read for months and even years to come, even though they may no longer accept new replies.
