I've been on SEO forums and newsgroups since the day dot. I remember the glory days and although I find most forums to be clique filled speculation pits of ego boosting trash - I don't regret the time I spent on them in the early days.
I am glad that I've kept, more or less, undercover. I give up a forum alias if it becomes too widely known and move on. I just like to keep my head down - which is why it's taken so long for me to start this blog, why I'm not promoting it and why it's covered with disclaimers and caveats.
I had to abandon one of my aliases as searchenginewatch's forums (which are, I think, probably the best forums today) as it picked up a whole whack of reputation. I'd been posting with this alias for months, generally seeking out those posts with no or only a few replies and providing as helpful an answer as I could. You don't win reputation for anything like that. I made the mistake of linking to an updated page within Google's guidelines and that was enough to spark around of votes and "keep up the good work" comments (which grates against my arrogance as they sounds patronising ).
My current alias is about as easy to guess as could be so I imagine I'll abandon it in a little while - or perhaps put it out to stud, perhaps for return visits when I want to make a post in a semi-anonymous way.
On the topic of undercover forum use we have Matt Cutts and GoogleGuy. For the longest time the wide spread belief was that Matt Cutts was GoogleGuy. This is still likely to be the case. However, Cutts has begun to cite GoogleGuy posts in "almost" the third person over on his blog. He'll say something like, "Over at forum X, GoogleGuy has posted to say ... ". Of course, this does not rule out that Matt Cutts is GoogleGuy. He could be referring to the 'forum alias GoogleGuy has posted' and still be grammatically correct. Is it 'evil# to lead us to believe something by implication? A little. Whereas Matt might have been safer to leave well alone, I can see that he might also want to draw as much attention to GoogleGuy posts as possible in those situations where he wants people to read them and his blog is a good vehicle for that.