Meaning, it's the hot new subculture that 'everyone' is curious about. It's got a core population of practitioners who have been doing it forever, but as the result of recent publicity there is a growing flood of newbies, a fair number of whom will try it and move on once the buzz dissipates. There are specialty shops where you can obtain paraphernalia and hook up with more experienced community members, and new venues are popping up rapidly. There are blogs. There are meet-ups. There is jargon. You see people on the street more openly practicing or displaying the tools of the trade.
However, one thing in particular makes me proffer this thesis right now: knitting has hit the programming circuit at events like science fiction conventions.
What does [BDSM, knitting] have to do with SF? Not much, beyond a reasonable overlap in interested population. Hence the inclusion of [BDSM, knitting]-related panels and activities at SF conventions (like, say Norwescon). Mind you, people practiced [BDSM, knitting] at conventions long before there were panels about it! But the interest has grown to the point that a degree of organization is starting to shake out of the matrix. A once-marginalized portion of the population is starting to emerge as a group, out and proud, forming new coalitions and boldly displaying the badges of their subculture.
I haven't looked for it, but any minute now I expect to run across a Knitting Pride flag. Perhaps this weekend.