Potentially, yes, subliminal influence of various kinds could represent some sort of social threat. It isn't likely that people could be turned into automatons by subliminal influence, but it is arguable that we are already greatly, even unfairly, being influenced by carefully crafted psychologically sophisticated persuasion tactics.
The development of new technologies, such as high resolution television, make the possibilitues even greater. The incredible amount of time most of us spend in front of a television creates a real potential for even a relatively weak message to become effective through sheer massive repetition.
The potential is not just that people might be persuaded to favor one product over another arbitrarily, or buy junk that they don't need. There is also a potential that our morals might be subtly usurped, and that the entire social and political structure could be influenced by swaying our voting preferences and decisions on important issues. This is by no means limited to "subliminal" influences ! It is true of all sources of information that we accept uncritically.
To the extent that we are already greatly influenced by mass media, the situation can only become more complex with increasing technology.
Are we currently being influenced subliminally ? Even James Vicary, the inadvertent founder of the subliminal movement, was amazed that people found his invention such a threat. Read more on the background history of subliminal influence to put it into better perspective. Vicary considered it to be most useful as a way to reinforce an advertisers message in a more efficient way, not as a way of changing buying behavior. This is closer to what researchers find in their subliminal priming experiments.
But remember that the public later came to use the term subliminal in a much broader way than applies to Vicary's device or subsequent image flashing devices. Many of the basic principles of influence discovered by social psychologists also fall under the wide umbrella commonly intended by subliminal as any hidden message or influence. These are definitely effective, and definitely widely used to persuade us.
The danger is not specifically from unseen messages. It is from the holes in our thinking that can be exploited in all sorts of ways. Unseen and unheard messages are neither the most threatening nor the most effective forms of influence that we should be concerned about.