Depending on the exact circumstances, I am not sure whether the placebo effect can completely explain any positive effects of faith healing (esp. since all studies I have seen document no positive effect at all or even a negative effect, see Frako's post). What I have read about these preachers that lay on hands in front of large crowds (which is of course only a small subset of all possible ways of faith healing) suggests that they focus on people who are psychologically unstable to begin with and who are therefore easily duped into believing that they have been healed. In this case, I would expect that any effect would be only temporary. And in many of these cases, it's of course just plain fraud.
Edited by Dirk, : typo
Edited by Dirk, : clarity