Way back in 1984, Ken Thompson wrote what I consider one of the greatest papers on computer security. I wrote about this in my The Greatest Hack of all Time post. If you haven't read this paper your education is incomplete and you should remedy the situation immediately. You can find the paper here.
Bruce Schneier is reporting that David Wheeler has published a paper on how to detect the Thompson hack. This is good stuff. The solution is simple and obvious in retrospect but interesting nonetheless. You really should take a look at this (and Thompson's original paper) even if you're not particularly interested in security—it's part of your heritage.