First, it is important to keep in mind that this poll was an internet poll and a voluntary poll. This means that the results may be skewed one way or another. Perhaps more scientists take drugs and don't want to admit it. Perhaps those who don't have a problem with mind-enhancing drugs or those who take such drugs would be more likely to take the poll than those who don't (I think this is probably quite likely). Or perhaps non-scientists "stuffed the ballot box" as a joke.
Any pollster worth the title will tell you that accurate polling is very difficult. If you ask 1000 random Americans "Do you approve of the job President Bush is doing?" and another 1000 random Americans "Do you approve of the job George Bush is doing?", "President Bush" will get higher ratings than "George Bush." The use of the title of "President" subconsciously instills a little more respect in people. Also, if you ask 1000 random Americans a question and 1000 random iPhone owners the same question, you may well get different results, because the typical iPhone owner is not identical to the typical American.
In short, before we make too much of the "juicing" scientists story, we need to realize that the respondants were a self-selecting group, and I do not know how the phrasing of the poll questions was chosen. So, it is probably wrong that 20% of scientists are trying to enhance their mental abilities. But I don't know how wrong. And the number might vary from science to science -- a high-pressure science, like medicine or genetics, might have a higher fraction of people willing to take a substance than a lower-pressure science.
Still, I think that this result should be pursued. First, it is interesting in and of itself. Second, there are ethical questions one can and probably should ask about taking mind-enhancing drugs. Should a person take a drug that might enhance their mental ability, even if the person is otherwise healthy? What is an appropriate trade-off between the mental enhancement and side-effects?
As for me, the only mind-enhancing drug I take is coffee, though I probably consume too much of that, especially when I am at the telescope. I have strong (but somewhat vague) ethical problems with non-theraputic uses of drugs. But I would argue that most of the science that we astronomers do is non-critical, in that people's lives don't hang in the balance. If I were working on cancer treatments or AIDS vaccines or similar science, I might feel differently. But since I have no experience in such work, I can't honestly say how I would feel.
Still, I think that we need to be careful messing with our minds. Our brains are amazing organs, the products of millions of years of evolution, designed and built to allow us to function efficiently in ways we have only begun to understand. Tampering with that could be asking for trouble.