Here's what Introduction to Psychology, 6th edition, by Dennis Coon, 1992, West Publishing Company, MN (ISBN 0-314-92211-3) says about subliminals:
(p89) Unpleasant stimuli impede the ability of the subject to recognise them (Erdelyi, 1974). The phenomenon, known as perceptual defense, apparently occurs as the brain filters out information that causes anxiety, embarrassment or discomfort (Dember & Warm, 1979). Conversely, subjects are more receptive to information if part of it is pleasant (Zajonc, 1980).
(p89) Any information that is processed by the brain below the limen (threshold of awareness) is termed subliminal (Coon, 1992). Existence of subliminal perception has been confirmed (Niedenthal, 1990). However, experiments have shown subliminal stimuli to be of weak effect; subjects are more strongly influenced by strong, clear attention-demanding stimuli (Coon, 1992).
(p89-90) Spoken messages recorded backwards into music (known as backmasking) were evaluated for effectiveness by Vokey and Read (1985). They recorded a variety of sentences backwards, including parts of Lewis Carroll's Jabberwocky and the 23rd psalm of the bible. Their experiments found no conscious or unconscious recognition of their meaning; they were unable to influence their subjects with the recordings (Vokey & Read, 1985).
(p316) Tapes are available that contain subliminal messages, purportedly to "subconsciously" influence the listener to ease pain, lose weight, advance their career, improve academic performance and so on. The messages are buried underneath ocean sounds, birds chirping and so on.
Two such tapes, Improve Study Habits and Passing Exams, produced by America's largest 'subliminal tape' company, were tested by Russell, Rowe and Smouse (1991). The psychologists divided test subjects into three groups, one which listened to the real product, one which listened to tapes with soothing sounds but no subliminals, and one which listened to no tapes.
The experiment, which lasted 10 weeks and compared average final grade results and semester grade point averages, found that there were no effects or benefits from listening to the tapes (Russell, Rowe and Smouse, 1991). Individuals who consider themselves aided by the tapes are apparently feeling a placebo effect (Coon, 1992).