In addition to the excellent answers given here, the following two articles may be relevant as well:
Old Trick Threatens the Newest Weapons. John Markoff, New York Times, October 26, 2009. (Discusses risks of hardware trojans. Quotes from a White House review which found that there had been several incidents of compromise of hardware, but provides no details. Discusses Crypto AG, a manufacturer of crypto gear, which introduced a backdoor into their crypto hardware at the request of the NSA. Mentions other cases.)
The Hunt for the Kill Switch. Sally Adee, IEEE Spectrum, May 2008. (Discusses the risk of hardware Trojans. Mentions speculation that Israel introduced backdoors into a Syrian radar site, which allowed them to bomb a Syrian military facility without being detected by the air defense radar.)
To learn more about the remarkable Crypto AG story, you can read at Wikipedia, the Baltimore Sun, reporter Wayne Madsen,
and elsewhere.