When is it not history?
Short answer: when you think that, by studying an aspect of the past, you can change the present.
ETA: For "history", read "doing history in the sense of studying and/or professing history" not "at what point does history, the study of the past, stop being history." After reading meg and NK's comments below, I realized I wasn't perhaps as clear as I should have been. This is not about history being a valid field of study ... it's more about what I see as the duties of the historian when actually doing history. I'm not saying that who we are and what our own interests are doesn't or shouldn't in some way influence our choices of subject -- if that were true, we wouldn't have social, intellectual, political, economic, institutional, whatever history. I'm saying that when we let our interests in those things in the now drive and guide how we read and interpret and publicise the past, we are in danger of privileging the cause over the subject or the approach.
Apropos of this post and the comment threads here and at AIR, I've done some thinking. Part of it comes back to a question that most of us deal with on a regular basis, especially if we are in the classroom or lecture hall -- how is this relevant? My answer is, increasingly, "I don't know. Why should it be? Shouldn't you decide how it is relevant to you?"
Not history. Social engineering that uses history. That's a different thing.
I can explain the relevance of History as a discipline. For me, at least, it accentuates many of my natural thought processes and encourages my inclinations towards skepticism and picking things apart till you think they can't be picked anymore -- then putting them back together a different way and looking for holes. The knowledge that my subjects are not like us means that I have to try to imagine what it would be like to be like them: it will always be an approximation, but I have to play by their social rules, which means I have to develop some empathy and understanding. Those skills are things I do use on a daily basis in the present when confronted with trying to understand current political situations, my fellow citizens, visiting other countries -- or even other parts of this country.
As far as why Eastern Francia or Stuart England or Lombard Italy are relevant? I can't help you, except to say that the stuff these people did happened within a greater time-stream that seems to have got us here and now (this is one of those places where the Histroy of Science wonks have it made -- there, you really do seems to be able to relate one event to the next in an idea of 'progress'). Some of this is conscious, some not. Some is based on reiterations of "what happened" that are more or less correct, some of those interpretations have now been corrected -- but we have to deal with the fact that the misinterpretations have had influence. All of this is meaningful, all of this adds to to a greater understanding of the world, but as far as personal relevance goes? That's up to you.
I think that, by learning to understand the past -- and that means knowing that it's the quest that's important, and that, even though we defend our conclusions with every shred of evidence available, those conclusions can be overturned with a single new document, or argued into a new interpretation, and that's a good thing -- we can learn to understand the present better. But understanding the past to change the present? Or to undo wrongs of the past in the present? I think that's where you cross the line and stop doing history.
Part of me thinks that this is particularly a danger for the modernist, or at least some modernists. Perhaps because some perfectly acceptable approaches for modern historians still allow for thinking of history as progess? But the minute one thinks of history as progress, one is almost forced to use the present as the standard of measurement, and that's not going to result in good history.
Perhaps more importantly, the recent past is accessible in a way that cannot help but to be personal. My own views of the Depression, World War II, and post-War Britain are colored by having heard the personal recollections of my grandparents, in-laws, and ex-husband. But I also know that those experiences, even though they've often been presented as normative, might not be. My favorite example is the story of my ex-husband's birth -- he was born in his grandmother's bed, as were his brothers. I asked if that were normal, because he was born in 1951, after all, and was told it was. And you know, it was normal for the people in their neighborhood, which was poor. But another friend was born in a different part of England, four years earlier, and to a different class. For him, the idea of being born at home was abnormal -- even his much older siblings were born in hospital. Intellectually, I of course always knew that these stories in part show -- that people of different social groups probably had different ideas of normality. But there is an unconscious temptation to give more weight to the person, to the eyewitness account, that one knows, and that makes for bad history.
I think that "identity X" history can combine the worst of the dangers I've mentioned. There is an idea of history with a goal, and there is trust of the personal account -- except in this case, the personal account is often the personal experience of the historian and the people closest to him or her. For what it's worth, I think that Americans who are historians of American history have to face many of the same challenges (as do Germans who do post war Germany, etc.). And the more I think about it, if you've chosen to do the history of something because you have a personal identification (in real time) with it and an interest in using it to make your own personal identifications explicable and meaningful, you really aren't doing history -- you're doing sociology or journalism or poli sci or genealogy, but whatever it is, you've crossed the line and compromised your objectivity. Because, if it's all about you, it isn't history, I don't think.
Update: Alun Salt offers his typically cogent views at Archaeoastronomy and Revise and Dissent.