I am actually woefully illiterate when it comes to feminist theory. I mean, I've definitely read more than the general population, but I'd say the majority of this community is better-read than I am. So, apart from her work with NOW, I'm really unfamiliar with Butler. Hopefully, this discussion will teach me more about her.
As far as I know, she's a postmodernist who takes the postmodern statement that language constructs reality and that language goes before reality, to gender. So: because we have the duality of gender in our system, we come to define bodies that way and then there will be female and male bodies, but only because of the way we deal with it in our language. Am I right to summarize it like that?
That's my understanding of postmodern gendering theory. I'm not sure if I entirely agree with it, although it does explain why many cultures (but especially Western ones) are so put off by intersexualism, androgyny, amd any other variety of people that don't fall into our (artificially) binary gender types and roles.