This is a question that is raised whenever the presidential candidacy of Barack Obama or Hillary Clinton is reported or discussed. I don't think it leads to an especially useful conversation, or at least, not in its current incarnations. This is because the conversation can be very speculative. It is often asserted that Americans harbor too much latent racism to elect a black, and a little less often, that Americans hold too rigid a definition of gender roles to elect a woman. My sense has always been that Americans today would elect a black or a woman because we are accustomed to them/us in most facets of our society, but I am by no means an authority on public opinion. I don't think many people are authorities, which is why I wonder whether this conversation can be useful. After all, if we decide that Americans, most of whom none of us know, are not ready for a non-white, non-male president, then we implicitly discourage the Obama or Clinton candidacies because "the time is not right." Even though we are ready for a black or a woman, they are not.
However, historical change, and especially change in favor of inclusiveness, must be pushed. Rosa Parks did not wait until the time seemed right to sit in the front of a segregated bus, and black students didn't wait until the time was right to enroll in U. of Mississippi. Neither Clinton nor Obama face the institutionalized discrimination of these earlier cases. Why then should we try to hinder their candidacies with unfounded speculations about what Americans are ready for? The best thing anyone who hopes that America is ready for a black or a woman or anyone of any race, religion, or ethnicity can do is to take that person as a candidate first and forgo the sheer speculation that often comes with attempting to diagnose our country's level of prejudice.