See Justin's explanation of the significance of this stance.
Also, read the essay by Robert George, which begins:
Barack Obama is the most extreme pro-abortion candidate ever to seek the office of President of the United States. He is the most extreme pro-abortion member of the United States Senate. Indeed, he is the most extreme pro-abortion legislator ever to serve in either house of the United States Congress.
Yet there are Catholics and Evangelicals-even self-identified pro-life Catholics and Evangelicals - who aggressively promote Obama's candidacy and even declare him the preferred candidate from the pro-life point of view.
...I have examined the arguments advanced by Obama's self-identified pro-life supporters, and they are spectacularly weak.