This article about tattoo influenced culture makes for a good read. I find it interesting that Sailor Jerry (one of my favorite clothing lines - available at Art-N-Body) runs revenues of about $16 million a year. This is much more than I would have imagined and is a positive sign of the times. Old skool tattoo imagery is still in and growing.
I agree with the author in that tattoos seem to go against the traditional bell curve of cultural trends. Typically as unique personal identifiers become more popular they eventually die off as a result of not being unique any longer. Even with the bulk of the population having tattoos - their popularity keeps growing. The uniqueness of each work of art may be the reason why. In other forms of fashion there can only be so many designs created until the brand or a specific design becomes vanilla. Tattoos are a fashion statement that are rarely reproduced. Even with flash art being shared most artists will put their own twist on a piece so that the design represents a unique personal piece of art. All of this being said tattoos are really not a trend as continued lifetime growth and positive law trends continue to push them into popularity.
The uniqueness of tattoos will come from the quality of the wearers artwork and the amount of work that they actually wear.