Really not convinced, whilst I don't believe they were actors, I don't think the placebo was quite as effective as Derren would have you believe:
1) The timid guy in both of the 'fights' was backed up by someone, in the first argument the cameraman was more well built than the 'drunk,' in the pub his mates were stuck, the guy who was doing the pub quiz sign up sheets seemed to be on the timid kid's side, and seeing as his mates were in trouble he probably felt he had no alternative.
2) The heights guy, the placebo probably did help him get over his fear of bridges, however at the end he was on a harness, I'm also scared of heights, however I've done Go Ape before, because whilst I was still pretty nervous the whole way through, I knew the harness was stopping me from dying, the guy is hardly going to say 'can someone take the harness off this feels like cheating,' because then he'd have to face his fear head on, and he's just been told the magic pill doesn't actually exist.
3) The smokers, they only said what was happening a month after, quite a lot of people can quit for a couple of months, I'd be interested to see how they were doing a year from now.
4) The intelligence people never got mentioned again, I'm assuming that the one person who realised it was a placebo told everyone else in the group so they had to void that group, although I have a theory that someone deliberately let slip to them it was a placebo, to make the experiment seem more realistic.
5) The allergy group, it is possible a placebo 'cured' them, it is well documented placebos can make people think they are getting better, that's why when new drugs are tested they always give 50% of the test group a placebo, to make sure it's actually the drug having the effect.
6) The singing person, it wouldn't surprise me if Derren deliberately made her fail the first time, we heard nothing about how much warning she was given before she started busking, based on how badly she sang in comparison to the end I would guess less than 24 hours, without mentally prepping herself she didn't really have a chance. The same could probably be said of the stage audition. In contrast she was given 'weeks' to prepare her song at the end, and whilst Derren said he hadn't told her what she would be doing, having already been thrown into 2 situations she could probably guess at what was coming, and not wanting to fail for a third time probably would have been mentally prepping herself, as well as getting practice time on the song so that she didn't end up sounding horrible again.