Look... there have been many legal cases over this issue. Let's say you're a public figure. If the circumstances amount to strong evidence that there is actual malice involved (which does happen sometimes), then the injured party can get information subpoenaed in order to find out who said it. Legally, it's all based on reasonable levels of evidence, not just some ability to willy-nilly subpoena anyone you want.
There was a case in the news not long ago, in fact it was mentioned here on Slashdot. (I'm not going to take the time to look it up, though.) After seeing the evidence, the judge ruled that the plaintiff was likely able to show genuine malice, and so agreed to issue a subpoena for information about who the individual was. If the judge had found insufficient evidence of malice, he would not have issued the subpoena.
In this case, though, the standard for breaching anonymity was that the accuser
stated as a matter of fact that the accused had committed a crime. See paragraph 26 of the decision, which is liked to from the story. I think that standard is far too low.
Here's a hypothetical example: the writer says, "I'm doing this because I think you're a low-life hateful/awful/dangerous person." (As opposed to, say, "I have evidence you've actually done something wrong.") Or "I'm doing this because you seduced my wife", etc. So for this example, a personal vendetta of some kind. If the statements are false, and the writer admitted that it was done for personal reasons, or personal gain of some kind, that's generally evidence of malice.
But again the point is: there has to be evidence before a subpoena my be legally issued.
A person who has inside information about nefarious deeds, but who is, because of his position, vulnerable to retaliation, should be able to speak anonymously.
They can. The law allows them to. What the law does not allow, is malicious, defamatory speech. As long as you're telling the truth, there is generally nothing to fear: truth is a near-absolute defense against libel. But if you're lying, and especially you're lying for opinion-based or selfish reasons, you'd better watch out.
That's the way it is. And frankly, that's the way it should be.
As long as you are telling the truth, and can prove it, you have nothing to fear from the law. There are other forms of retaliation, though, such as losing your job or being shunned by your friends. I don't think Deep Throat would have spoken out about Watergate if he could not have been assured of anonymity. By the standard that this court applied, Nixon could have filed an action to seek his accuser's identity, since he was accused of breaking the law.
If an anonymous accuser provides evidence enough to prove that his allegations are truthful without his testimony, then there is no need to unmask him. If he doesn't, he can be ignored.
I think that the cost of being lied about anonymously and not being able to punish the liar, is less than the cost of being unable to accuse a powerful person of having committed a crime while remaining anonymous to avoid retaliation.