It would be nice to think there is enough racial mixing for us to envision a "post-race" world in another century, but I very much doubt that is the case, and even if that were to occur, geographical evolution would eventually sort us out in some way that would make people different again.
From a genetic perspective, how racially mixed people will look is rather difficult to predict. Think about the people you know who have parents from different races. The physical characteristics are likely to be quite different in each, and each is a unique blend of the genes of the parents. My own children, who are biracial, are a good example. People can tell they are brothers, but they have very different appearances.
And even if we all become completely mixed, with no discernible racial characteristics at all, we must all eventually adapt in some way to the environment in which we live. If we live far from the equator, our skins must get lighter, so that we can absorb more vitamin D when we do get some sun, for example. In spite of the fact that we speak about a global village, in order to have civilization, most people need to be geographically "fixed," which means adaptation to one's surroundings, with those best adapted surviving to hand down their genes.
On another note, racial differentiantion is largely a social construct, not a biological "fact." The DNA of an African-American person can have more in common with the white person across the street than with another African-American person. Perhaps we can all live in a post-racial world if we come to understand this. I have included a link to an article on this topic.