I am neither prone to conspiracy theories nor particularly fearful of the NSA. However, I believe that the last three months have demonstrated that previous answers to this question were naive. Rootkits on computer controllers and/or BIOS/EFI boot code either already are or are about to become common. The most recent http://it.slashdot.org/story/15/03/19/1319244/persistent-bios-rootkit-implant-to-debut-at-cansecwest suggests that much. Once a hacker has root access on a machine---and the NSA [or clever and determined hackers, with or without the consent of the manufacturer] may obtain this access even before the computer even ships---all bets are off. The hard disk, the BIOS, and multiple chip controllers are all compromisable.
Such exploits are not just theoretical, and it will not just be the NSA, the Equation Group, or any number of foreign services that have them.
Instead, such rootkits will be panaceas for common criminals. The potential for extortion is there--at least the NSA is not primarily in the extortion or commercial spying business. This will get far worse very quickly.
If anyone knows of computers, motherboards, and/or hard drives, whose controllers are based on old-fashioned ROMs instead of reprogrammable EEPROMs updatable in the field, I would love to learn about it. It would be a prudent security feature, and couldn't come soon enough.