I came across an article on author Malcolm Gladwell ("Blink" and "The Tipping Point") that led me to his online archive of articles from The New Yorker magazine. There I found an interesting article on employment testing -- specifically, the role of the personality test in the selection process.
Personality testing is a $400 million-a-year industry, and there are more than 2500 kinds of tests out there.
Gladwell points out that one of the fundamental flaws of personality testing is in individuals' lack of consistency:
We have a personality in the sense that we have a consistent pattern of behavior. But that pattern is complex and that personality is contingent: it represents an interaction between our internal disposition and tendencies and the situations that we find ourselves in.
Gladwell's articles are often quite lengthy, but worth the read. I'd recommend this one in particular.