Can You Argue From Ignorance?

There is a standard reasoning error called "argument from ignorance" which is asserting that a lack of proof for something is proof of its falsity. For example, before I was about to teach this fallacy for the first time I came into the room to find the students engaged in an argument about the existence of God. The last thing said was "You can't disprove God's existence, so He must exist." This student turned beet red in the middle of class.

We ought to believe that for which we have good evidence, and that we do not now have evidence does not mean that such evidence doesn't actually exist, just that we don't yet know it. The proper stance of the ignorant is suspended belief, not to believe one way or the other.

The way one usually commits this error is by shifting the burden of proof. When you make a claim, it is now your responsibility to provide good reason for me to believe it, it is not my job to provide reasons why it is not true. Arguing from ignorance often occurs when you shift this burden of proof to me, where it does not belong. You made the claim, you support it.

But we do infer from ignorance all the time and sometimes it does seem proper. When someone pleads the fifth, for example, we generally take this as good reason to believe he is guilty. From a lack of evidence given to clear his name, we assert that we have a reasonable belief that he, in fact, did it. The lack of evidence of his innocence is what we base our assertion of guilt upon. But isn't this a good inference?

In this case, it is a shifting of the burden of proof, but in the context don't we have reason to suspect that this is a burden that he would gladly take up. We have reason to think that given the consequences of belief in his guilt, that he would do everything that would help him convince the jury of his innocence and not taking the burden -- even if if isn't required of him -- is odd and that oddity is best interpreted as likely guilt. He didn't explicitly tell us he was innocent because he probably isn't. He didn't explain away the charge because he can't.

Is this legitimate reasoning? If so, when is an argument from ignorance acceptable and when is it fallacious?