As an untrained outside observer (but one who has been thinking a lot recently about the statistics of causality) I'll chime in to say that I'm siding with 'ashearer' here. As I'm reading it, your approach doesn't make sense to me. If you want to distinguish between two treatments (placebo=treated and do-nothing=untreated) you need to measure outcomes for both treated and untreated individuals, and these individuals need to be "exchangeable" to some degree.

Perhaps when you say in the first post "give people placebo", you mean "give the placebo to some subset of the individuals leaving the remainder untreated"? I agree that you can often draw useful conclusions from an observational study as long as some comparable individuals receive each treatment, and as long as you can control for the initial differences between the treated and untreated. But if you are saying that you don't need to look at the results for the untreated group at all, then I think you are mistaken.