Here's a decent summary for criminal procedure at the federal level (from the Minnesota US Attorney's office):

http://www.justice.gov/usao/mn/criminal_proc.html  

Anth0ny, you're right, civil litigation and criminal law are different.  But, subpoenas are common to both, but I suppose they can be used differently.

EDIT: I conducted a little research on this issue. Two things: we've only heard about a "subpoena," not a "Grand Jury Subpoena." I believe persons suspected of a crime and that are being investigated for said crime may need a Grand Jury Subpoena to be compelled to testify at a hearing. Grand Jury Subpoenas of a target in an investigation require the approval of the grand jury itself as well as a US Attorney or AG.  

From the US Attorney's Criminal Resource Manual (Chapter on Grand Juries, Section Subpoenaing a Target of an Investigation): "If a voluntary appearance cannot be obtained, the target should be subpoenaed only after the grand jury and the United States Attorney or the responsible Assistant Attorney General have approved the subpoena."  

Here, it sounds like MK or Gox got served with a regular subpoena (but I guess we don't really know). If that's the case, I think my initial point still stands - either there are charges against them or its for somebody else's case. But even if the subpoena is a Grand Jury Subpoena where MK or Gox are the targets, there would have been some kind of proceeding first to secure the subpoena since the Grand Jury's approval is required. In that case, my statement about "charges" being a requirement is not accurate.