I wonder if the poly hippies tend to be older.. (i.e. 45+years old).
I've met other interest groups in the poly community. Wiccans, sci-fi fans and BDSM seem to be cropping up regularly. SCA (Society for Creative Anachronism) also is a common factor.
And this applies to the group, in that ... if you are hanging around these groups, you are more likely to find people who are accepting of the poly lifestyle.