Perhaps poly flourishes in BDSM relationships for a couple of reasons. 1) we're a little more open minded when it comes to relationships anyway - I mean Hell, we consider a lot of things "normal" that other people just flat out don't. 2) There are already defined pecking orders in BDSM, whether it be a one on one relationship or a poly. The key to either BDSM or vanilla, however, still seems to be very open communication and clearly defined expectations.

There is currently a thread called "Polyamorous or just not sure?" going on here on the forums, I would say check it out - lots of good thoughts on this topic.