Blacks are something like 10% of the total electorate, but a much larger percentage of the Democratic party (who are the main voters in the primaries so far). The other major party, the Republicans, has not been very successful in getting much black support since the 1960s. The Republican weakness is driven by a combination of strong Democratic support for civil rights laws (that's a simplistic statement, some southern democrats were very hostile to these, but is overall true) and sympathy to social programs, plus a conscious decision by Republicans, starting with Richard Nixon, to court white voters by playing to resentment and fear of blacks by pushing tough-on-crime, abuses of social welfare systems, and resentment of civil rights laws (look up the Wikipedia article on "Southern Strategy" to read more about this).
In the general election, the percentage of black voters is much lower, and Obama may have a much tougher time. There appear to be a lot of white voters who otherwise would support him, but who will oppose him simply because he is black. And Obama hasn't done well with Hispanic voters, who as you pointed out are one of the fastest-growing sectors of the electorate. I'm not convinced Obama will win the general election, though it is a definite possibility.
The Democratic primary also appears to have "activated" a strong segment of feminist voters who may well try to push a female candidate in the future, whether Hillary or someone else.
I'd say it is wide open, but will definitely be more diverse in the future instead of 100% white male. But white males (of which I am one!) aren't finished by any means. I'd be willing to bet that for at least the next fifty years, there will be more white males elected president than all other demographic categories put together, even if we do end up with two Obama terms to start it out.
For truth-in-advertising, I'll mention that I am a white male Republican-turned-Democrat-about-ten-years-ago, and Obama supporter, but I think the above analysis is a reasonably neutral one.
