In one board I used to moderate, when the person was banned, their status was automatically changed to "guest" while they were banned. That way people knew they were banned. I, personally, wouldn't want my banning to be made public, but I am a more private person when it comes to those things. Others are not, and I understand that.
I think there is a thread the MacUser started a few months ago, about celebrating those who were banned. I don't remember the name of it, though, but I know it exists.
merm:
I do agree there are positives and negatives for this information not being public. One negative for me is that it encourages gossip as to the reason the person was suspended, and allows people to hypothesize about the reasons for it. Often this gossip casts the moderators in the worst possible light. But, if an individual who loses their cool doesn't want that information public, currently we will honor that request despite the negatives.
I know for certain that it encourages gossip, both as a board administrator (seeing users gossipping in public threads), and personally (I've been witness to friends of mine gossipping about a ban - I'm not saying whether or not it was on this board, as I've been part of many virtual communities for over a decade).
I do believe that people are curious and want to know what's what, so in some ways it's natural that some people would want to know about the bans.