It could be an interesting discussion about what "kind" of woman or Hispanic or African-American would be a likely successful candidate.
One of the prejudices against a woman President would probably be whether or not she was "strong enough" to be Commander in Chief. I'm not advocating that prejudice; I'm just saying it likely will be there. Margaret Thatcher obviously didn't have much problem in overcoming that prejudice in Britian. Among prominent women in this country, Hillary Clinton and Condi Rice may be in the best position at the moment, and they would still probably have problems with the prejudice. A woman would have to be considered to be strong without being strident, an unfair standard, but one that probably would still be there. RBA mentioned Barabra Boxer, but I can't see her ever having enough moderate appeal to grab a few of the red states a Democrat would need to be elected.
A successful Hispanic or African-American candidate would probably have to be non-threatening to white voters. I'll use Barry Goldwater and Ronald Reagan, both of whom were conservatives, as examples. I realize some here probably hated both men, but Goldwater lost big and Reagan won big in Presidential politics in part because Goldwater appeared to be threatening while Reagan appeared to be a sunny optimist. Barack Obama has a much better chance IMO of being elected President than Jesse Jackson ever did because Obama gives the appearance of being an optimistic unifier while Jackson was polarizing. Colin Powell had appeal as a possible candidate because he appeared to be competent and non-threatening. One of the things I liked about Bill Richarson as a possible candidate was that he seems to be quietly competent.