Subliminal perception is a deeply ambiguous concept, which is perhaps appropriate if you think about it!
This is partly because the concept caught on originally as the result of a hoax. The now infamous "eat popcorn, drink coke study" was a planted story by marketing consultant James Vicary which became a wildly successful urban legend about selling products subliminally in movie theatres. Vicary's model wasn't entirely impossible in theory, but as he later admitted, he made up (or exaggerated?) his results to sell his consulting services and seemed honestly surprised that people got so excited about his planted news stories. He thought of it as a rather subtle effect and the concept caught fire beyond his wildest expectations.
It is also partly because of how much the concept has been abused to make the self-help industry more lucrative. It seems like less work and more exotic to listen to music rather than repeat affirmations or use suggestions, and people like less work and are attracted to the exotic.
Of course part of the ambiguity is because we are naturally fascinated and afraid of something that can potentially influence us without being aware of it. All sorts of paranoia (and also some justified fear!) results from this, especially since it is at least theoretically a real possibility. We don't trust many people who might have a technology like this, and for good reason.
Finally, it is partly because there are legitimately difficult technical issues to resolve within the perceptual research.
Some thoughts on the technical issues in subliminal perception.
First of all, it is important to understand that subliminal perception is real. Sometimes the term "marginal perception" is preferred because the concept of a single sensory boundary is not really accurate. In either case, these terms refer to the theory stating that perception can occur without conscious awareness and have a significant impact on later behaviour and thought.
Second, most of the research on this has been on visual perception, especially using rapidly flashed images and masking stimuli. Auditory subliminal perception is not as popular in experiments for two reasons. First, it is technically much more difficult to control what is going on in order to produce repeatable results, and second, because of those problems and the lack of interest in doing real auditory subliminal research, there have been no convincing repeatable demonstrations yet that there is an auditory subliminal perception effect of the same sort as in the visual experiments. That's why the claims for auditory self-help subliminal programs are not based on any legitimate research, because there really isn't much, and because practical tests have all turned up no evidence of effects beyond placebo. Enough on that. You can check the archives of any good Skeptic magazine to find the research by Phil Merikle and others that tested many of these products.
Third, getting to the legitimate phenomenon and the technical issues, the biggest ambiguity is in how we think of perception. Is it an all-or-none phenomenon or a gradual shift between different levels? This is an important question because it tells us whether effective stimuli can really be hidden from awareness or "unconscious," or whether they just fall into the shadows of our mind but we are dimly aware of them.
This isn't as easy a thing to test as it might seem. Different experimental situations produce clear but different results, and sorting out the overall picture is an interesting but challenging job for theorists. This article from Consciousness and Cognition suggests a perspective I tend to agree with, that if some experiments show gradual levels of awareness and others show all-or-none, then the underlying phenomenon probably consists of levels. It's easier to come up with a model of how levels can become all-or-none under some conditions that a model that creates gradual levels out of an all-or-none effect.
There is a recent experimental tool called the perceptual awareness scale that has been useful so far distinguishing whether something is actually perceived or not, and to what degree. Its implications for understanding subliminal perception are described in this article from Phenomenology and the Cognitive Sciences.
This is just a quick intro to a very deep and very tricky topic that has fascinated me for decades. The choice of the two articles which share an author was not a coincidence, I was inspired to write this after checking in on Thomas Ramsoy's work after I noticed that his wonderful Science and Consciousness Review site had been down for a while. I hope it comes back up in some form, it was once one of the best resources for research news on mind and brain topics.
Also posted at: http://www.scientificmindcontrol.com/forum/topics/subliminal-or-marginal