For the record, this post isn't written in response to anything that's happened on this site. Barring anything unforeseen, I don't expect to in any way hamper anonymous commenting on this site. And while I know theoretically it's possible to track down someone's identity via their IP address, I neither know nor care to know how to go about that.
But I do wonder whether or not people have any reasonable expectation of maintaining their anonymity when they comment on someone else's blog. I fully acknowledge and respect the right of people to start their own blog anonymously. There are many people who, for a variety of reasons, either couldn't or wouldn't write if their identities were publicly known. I read many of these bloggers myself, and I know the blogosphere is better for it. But that's at their own place. What about at other blogs? Is the blog-host who threatens to expose an abusive, trollish, or otherwise hostile anonymous commenter completely out of line?
On the one hand, it is obvious that not guaranteeing anonymity would deter certain people from commenting -- most notably the aforementioned people who blog anonymously. While I actually know of some anonymous bloggers who leave comments under their real name (but who don't link their name to their anonymous blog), many would be uncomfortable even with that. And furthermore, this strategy still blocks participation by their anonymous personas. A comment by "PG" is more meaningful than one by "Pam Gomez" (name deliberately chosen so that it has no chance of being anything close to PG's actual name, which I don't know), because even though they're the same person, I can link PG to her broader array of work, while I have no idea who Pam Gomez is.
On the other hand, anonymity has its fair share of problems. For one, if my blog is my house, I kind of have a right to know who is visiting -- particularly if they are slinging muck my way. People undoubtedly behave worse when they don't feel accountable to their real-world reputation. And, stepping across another line, if someone is being stalked or harassed online, anonymity can be very dangerous. Some blogs which depend on their commenters for credible, inside information (like Brian Leiter's) often don't allow anonymous comments at all because they aren't as trustworthy.
I think the norms of the blogosphere have developed so that, unless specifically told otherwise, an anonymous commenter can expect that her anonymity will be preserved unless specifically told otherwise. I do, however, think it is within a blogger's rights to warn an offensive commenter that, if she returns, her anonymity is forfeit. My house, my rules, so long as I give fair notice.