I'm guessing he's referring to the possibility of putting code in one of the following places:

  motherboard BIOS (http://arstechnica.com/security/news/2009/03/researchers-demonstrate-bios-level-rootkit-attack.ars)

  other peripheral BIOS (keyboard anyone? http://www.semiaccurate.com/2009/07/31/apple-keyboard-firmware-hack-demonstrated/)

  additional hidden attached solid-state storage

etc...

Although, this does involve touching the hardware, just not the harddisk so perhaps I misunderstand also.

http://blog.ksplice.com/2010/10/hosting-backdoors-in-hardwar... is interesting