See this thread on E-Budo; the article is here (for $2.95).
* 18.1 million Americans participated in some sort of MA at least once in the past year (9.4 million adults, 5.5 million teenagers, 3.2 million kids)
* 5% of adults say they participated in martial arts last year, and 28% of those say they do martial arts "every chance they get." This group is fairly evenly split among men (52%) and women (48%).