Placebo has strength in numbers (Mind Hacks):
Because the statistical concept of placebo is drawn from the study data, the study itself has an effect.
For example, the strength of the placebo effect is measured relative to the active treatment. The Wired article says that placebo is getting stronger, which is another way of saying that the difference between placebo and the drug is getting smaller.
It turns out that the more rigorous the study the less strong the drug effect is, or, in other words, the stronger the placebo effect.
For example, we know that better designed and higher quality studies show smaller drug effects. This includes things as simple as randomisation. If your method for randomly allocating people to groups is more susceptible to bias, it's more likely to produced biased results. Better randomisation improves the placebo effect, again, nothing to do with expectancy or belief.
So one reason why the placebo effect might be increasing is that studies are just more rigorous these days.A bit of a long clip, but there's more there if this subject interests you.