How might Wikileaks change the challenges facing future generations of historians ?

Prof. Skinner
According to Prof. Kiron Skinner, Wikileaks will result in fewer and less informative documents reaching the archives on which historians rely.

Skinner can speak with some authority on this issue. She is the co-author of “Reagan, In His Own Hand” and “Reagan, A Life in Letters.” She is on the advisory board of the George W. Bush Oral History Project and is the W. Glenn Campbell Research Fellow at the Hoover Institution at Stanford University. She also is an associate professor of history and political science at Carnegie Mellon University. Her government service includes membership on the Chief of Naval Operations (CNO) Executive Panel, the U.S. Defense Department’s Defense Policy Board, and the National Security Education Board. She has cochaired the CNO task force on the Middle East and currently cochairs the task force on the new Africa Command. She also serves on the boards of the Atlantic Council of the United States in Washington, D.C., and the World Affairs Council in Pittsburgh. Skinner is a member of the Council on Foreign Relations in New York City.
She writes that:
Governments require monitoring, but exposures such as those by WikiLeaks and similar outlets may produce unintended consequences. One result may be that fewer records are kept of diplomatic activities, thus curtailing, rather than advancing, transparency and openness. Another is that WikiLeaks and its imitators will transform the study of history.
She also suggests that historian may be forced to rely more on oral history, which is a problematic source.
as policy makers, intelligence analysts and statesmen find it necessary to write to each other in code, future students of history will find it a daunting task to decipher the records of past heads of government.
They will be forced to rely on the memories of living subjects or on real-time journalistic accounts. These sources — research mainstays for many historians — must always be cautiously weighed against archival documents. Eventually, however, those archives may become so difficult to use that they will slip into irrelevance. Constructing the past will be especially difficult in the absence of candid, carefully written historical accounts.
You can read the rest of her post here.
Skinner makes some interesting points, but it may be that Wikileaks has less of a long-term impact on the archival record than the advent of new technologies. I know from historians who work on post-1945 Canadian history that researching government decisions after the mid-1960s becomes a challenge because there is less correspondence in the archive due to the falling cost of long-distance telephone calls. Instead of a nice letter outlining the reasons for a decision, the archival record contains a short notation that a decision was made after a phone call.
The yellow Post-It Note, which first went on sale in 1980, has also discouraged decision-makers from recording their thoughts in letters. In some cases, civil servants will omit a key sentence or idea from a letter and include it in a Post-It note if they think there is a chance the letter will someday be the subject of a Freedom of Information request: the recipient of the letter throws away the potentially incendiary Post-It before the letter is filed away.
In a 2010 lecture on document disclosure to the Friends of the (British) National Archive, Jonathan Sumption, who is a practising lawyer and an accomplished medieval historian said he had asked senior civil servants about their record-keeping practices. He reported that:
With one exception, every one of them admitted to having omitted significant information from internal documents, which in earlier times would have been included, and to having communicated them informally instead, so that they would not be recorded in writing. One of them remarked that in some departments it was quite common for politically sensitive matters to be omitted from documentary records and recorded only on marginal notes written on Post-It stickers, which could be removed and binned after the right people had seen it… see here

The controversy over Wikileaks should be considered alongside the ongoing debate in China over Yang Jisheng`s new book Tombstone (Mubei), a groundbreaking work on the Great Famine (1958–1961), which, though imprecisely known in the West, ranks as one of worst man-made disasters in history. Yang was recently interviewed by Ian Johnson from the New York Review of books about the sources he used to research this book. The really interesting thing about Yang`s book is that it is based on archival materials: Yang used his contacts around the country to access local Communist Party archives and uncover more direct proof of the number of dead, the cases of cannibalism, etc. Johnson asked Yang why these records, which document the results of Mao`s terrible policies, were not destroyed.
Here was his answer:
Destroying files isn’t up to one person. As long as a file or document has made it into the archives you can’t so easily destroy it. Before it is in the archives, it can be destroyed, but afterwards, only a directive from a high-ranking official can cause it to be destroyed. I found that on the Great Famine the documentation is basically is intact—how many people died of hunger, cannibalism, the grain situation; all of this was recorded and still exists.
Yang was a journalist who worked in China`s state-controlled media for decades. Even more astonishing than his access to the archives was the fact Frank Dikötter, a professor at SOAS in London was able to see similar archival materials for his recent book on the famine.

So why were these documents left in Chinese archives so that Yang and Dikotter were able to access them ?
It seems to me that there might also have been another factor at work. I read somewhere that the archival record in dictatorships can often be more complete and easier to access than similar records in democracies, where government officials tend to destroy documents for fear they will be leaked to the press or result in a civil liberties court case. In a country without a free press, you don’t need to worry about such things. Moreover, the murderous thugs who staff the bureaucracies of dictatorships are probably less embarrassed by documents reported human rights abuses than civil servants in democracies are. In Canada, public confidence in the RCMP was damaged in the 1970s by the revelation that its intelligence agents had broken into the offices of a separatist political party in Quebec to look at their records. Needless to say, the RCMP tried to keep these activities secret. In a totalitarian state in which millions are routinely murdered, a document recording the details of a single individual`s suffering may be perceived as less incriminating than it might be in the West. There is less of a shame factor to prompt officials to destroy documents. The Nazis, after all, carefully documented the Holocaust even though they were incriminating themselves in the process. Admittedly, their leaders weren`t expecting to lose the war and be put on trial in Nuremberg, but the very fact they generated evidence of their crimes suggests a total lack of shame.

Letter by Reinhard Heydrich to the Head of the SS Personal Main Office SS Gruppenführer Schmitt, January 25, 1942 conc. "Final solution of the Jewish question" (in German)