Some secrets should (morally) be kept. Others should be exposed. The question for any specific secret is, of course, is whether it belongs in the first or second category.
Naturally, I am thinking about the WikiLeaks leak of the information about the war in Afghanistan. However, this can also be generalized to all such leaks.
Some government secrets do seem to fall into the first category. For example, leaking the names of undercover agents would seem to be wrong. Other government secrets fall into the second category, such as misdeeds being carried out by politicians or crimes being committed by officials.
Naturally enough, the folks in government tend to insist that all their secrets fall into the first category. They invoke national security and so on to keep things in the shadows. As involved parties, they can hardly be expected to consistently reveal what is really going on. As such, there does seem to be a real and legitimate need of leakers (and gadflies, as Socrates would argue).
As far as justifying leaks, the following reasons can be given.
First, in a democracy the citizens seem to have a right and need to know what is being done. Otherwise, they cannot make informed choices when voting. Also, they have the moral responsibility to be aware of what is being done in their name. If the people in government are unwilling to provide this information, then leaking would be justified.
Second, people in power have a natural tendency to seek to conceal what they are really doing and a desire to avoid criticism. This was true in Socrates time and is true today. Since these people are generally not inclined to reveal the truth, then leakers have to ensure that the people know what is going on in the shadows of their government.
Of course, there is a serious concern here: what should be leaked and what should be kept secret? In short, what do the people have a right to know and what is right to conceal?
In this case, I would go with Thoreau-the individual must examine his conscience and the information and make a moral judgment as to what to do. What she decides might be the right thing or the wrong thing, but that is true of any moral decision. To say that people should always rely on the judgment of those who are keeping the secrets is as mistaken as saying that all secrets should always be leaked.