Personally, I think we're more likely to see Oprah win the presidency than Nader. He's a sharp guy, but he'll never appeal to the overall public enough to get any real attention.
Oprah runs, she WILL win, because the voter turnout rate is so low for the other two parties- she'll get millions who would otherwise not have voted at all (mostly bored housewives who neither know, nor care, about politics.)

Although, yeah, I would vote for Obama. Or Perot. Or Nader. In THAT order, if any of them ran at the same time.