One of my pet peeves is when people confuse the fallacy of authority with the argument from authority. The former is better called the fallacy of irrelevant authority, since it's when you use the views of someone who is an authority in a particular subject to justify positions outside of that subject. It's the irrelevant part that makes it a fallacy, not the authority part. For example, I often see letters to the editor, editorials, and essays where the writer uses his status as an academic to promote his political views, even though his academic standing is in a field completely unrelated to political science. Such a person's political views carry no more weight than yours or mine or anyone else's.
By contrast the argument from authority is a perfectly valid form of argumentation in informal logic. It takes as its premise that an authority on a subject is more likely correct about that subject than not. In my opinion the most it can do is shift the burden of proof to one who is denying or arguing against the authority's position. Also, in order to have any strength, I think it has to be an argument from the consensus of authorities on a subject, since you can always find some authority in some discipline willing to make outrageous claims.
In formal logic, however, arguments from authority are invalid. This is because the validity of a syllogism is completely distinct from the person presenting it. However, again, some people think this means that any appeal to authority is invalid, regardless of whether it falls into formal or informal logic. For example, I've actually had discussions with people who think Jesus never existed, and when I mention that their view contradicts those of virtually all historians who have written on the subject, they yell, "Appeal to authority! Invalid!" Of course it's not invalid at all. It's perfectly appropriate to look to those who have devoted their lives to the study of the subject under debate to see how they have assessed the evidence. Again, I think it only goes so far -- it's a weak argument -- but it's still a valid argument, as long as you're working outside of formal logic.
(cross posted at Quodlibeta)