I wondered about this too. I was at this horrible munch over the weekend and when I left several people encouraged me to check out a local dungeon, which I said I'd think about, and then one guy asked if I was a top or a bottom, and I said, I'm submissive. As I left I thought about this.
First, given the distinctions that have been made on this thread, I clearly am submissive, but also, for me the pain or sensation aspect is not really thrilling in itself, without the aspect of submission and pleasing another person. I couldn't imagine - at the moment, I seem to be in a very fluid time of my life - having someone hurt my body for MY pleasure.
So I wonder, does "bottom" imply enjoying the physical sensations of s/m interactions, or did this just arise in the context of my conversation because I was, after all, being invited to a dungeon? Maybe "sub" allows for enjoying the D/s dynamic without implying a pure physical enjoyment of every sensation?
I know that in another thread there was a discussion of when a Dom might choose to bottom in a sexual interaction, or when a sub might be allowed/required to top.