Boing Boing references an essay by Pam Noles about the lack of race in science fiction and fantasy.
I think Noles has a real point, but she (perhaps intentionally) avoids one of the implications of pulp-derived literature like SF. While there isn't, generally speaking, a lot of variety in human race in the genre, aliens/monsters/metahumans often stand in for other ethnicities. They undergo discrimination, taboos against miscegenation, racial stereotyping.
That doesn't excuse the lack of non-White people, in my opinion. In fact, it worsens the situation. When authors use that kind of metaphor to examine real-life racial tensions, it also carries the message that there's something non-human--or less than human--about anyone who's not a Caucasian. The common use of stereotypical attributes to define aliens and monsters (like Tolkien's noble elves and hateful orcs) only worsens the problem--I'm sure other people have noticed that the violent, brutish, sexually aggressive Klingons of Star Trek fame were also darker-skinned, practically parodies of Black stereotypes. Eventually, if I remember correctly, even the show's writers noticed. But by that point they were hamstrung by 30 years of backstory.
There's a lot of room in science fiction to explore these kinds of contemporary issues, which is why it can be so frustrating to read "post-human" books which dismiss and jump past our reference frame. It's unfortunate that the tropes of the genre have made it so easy to explore them badly.