BDSM: Authority, equality or what?

I was reading a post the other day on a blog I follow, Obedience with Grace, who is a slave in an M/s relationship wherein she was discussing the notion that there is no such thing as equality in any type of dominance/submission relationship and that submissives who feel they are equal to their masters are wrong.  This got me thinking and I replied to Grace with the basic idea that equality is not relevant in BDSM; it's about authority and role.

I've continued to think about this since then...