I find it interesting that they have given such a detailed description of how they purportedly got this information.
On the one hand, it gives some credibility to the story since PW could presumably easily prove or disprove it. On the other, it seems stupidly reckless to have disclosed this description, since, if true, it would make it infinitely easier to catch the culprits before the expiration of the deadline.
It also seems catastrophically stupid to have sent these USB drives to so many people if their real objective is getting paid - the Romney camp would have to be insane to believe that they won't release the returns whether they get the ransom or not. Doing this also provides yet more physical evidence which the FBI/Secret Service/etc. can use to track them down.
On balance I don't believe this intrusion and theft ever occurred.