There are two main methods of distribution. Fileshare, where direct links to a copy of the file in question are distributed and torrents where hosts of files can be downloaded to your machine while you simultaneously share what you are downloading with other people, spreading the word. Links to the latter take a while to fully propogate through the system, the first maybe arriving late on the Wednesday that comic books ship, mroe on Thursday and generally the lot by the weekend.
But the direct downloads are faster, and more furious. I discovered that Chase Variant had been pirated and was available for direct download by 11am EST the Wednesday of publication. Now that is fast.
But but appears that one of the fileshare systems has been cut off at the knees. File server Rapidshare based in Switzerland has been banned from distributing copyrighted content, in a case brought by a number of book publishers (but not comic book ones)
The court ruled that Rapidshare must monitor its site to ensure copyrighted material is not being uploaded and to prevent unauthorized access to the material by its users. The company will be subject to substantial fines for noncompliance.
Of course this does mean that Megaupload, Hotfile, MediaFire, Sendspace and Oxiedon have the field to themselves, after losing their biggest competitor for this kind of material.
Until the courts come for them as well, of course.