Barack Obama delivers his presidential nomination acceptance speech tonight, not at the convention center in Denver, but at the Mile High Stadium there, with some 75,000 people in attendance.  And as he does so, he becomes the first black man to head his party’s ticket, 45 years to the day after Martin Luther King delivered his ”I Have A Dream” speech here, in Washington.  Obama is expected to talk about how, as president, he would make a difference in people’s lives.  Last night, his chosen running mate, Joe Biden, accepted his nomination.  NPR’s David Gura has more on that.