I completely and whole-heartedly disagree with Steve's take that people should be allowed to delete their comments in communities. There's this thing we all do called "making a mistake". And there those of us who are adult enough admit it, learn from it and move on. The goal is to not repeat it.
Then there are those who play the PR card by ignoring it, discrediting the other party or try to remove it. W-T-F, indeed.
There's also this thing that we call thinking before we speak. Could some of the things I've posted on my blog come back to bite me in the ass? Sure. Are they controversial? To some, yes. Am I blogging because I'm giving my opinions? You figure that one out for yourself. But that's how you build trust in your readership - by being genuine, not having a strategy on how to influence people.
And if someone asks me about something that I posted in the past I'd not only own up to but explain my position and engage in that conversation. That's what happens in communities.