Steve Silverman's interesting article in Wired about how pharma companies are beginning to research the placebo effect directly has been making the rounds in the 'sphere. Robin Hanson is skeptical of their motivations and Vaughan speculates that the statistical placebo effect might be growing because studies are more rigorous these days. Seth Roberts pointed out last year that, "If you get better from a placebo effect, that’s the wrong reason. How dare you!"
Placebo is sometimes touted as a mysterious effect, but really it's under the larger psychological umbrella of reward expectancy. Hypnosis is another example. Predicting which people will be susceptible to hypnosis based on personality traits does show some weak correlations with agreeableness. But by far the best predictor of whether or not hypnosis will be succesful is whether the individual undergoing it expects to be hypnotized. People's responses to alcohol are yet another example. Why else would there be so much cultural variance in our behavior while drunk?