Is the U.S. still a racially segregated society? If not, does that mean that racism is no longer an important issue in the U.S.?

Legally, no. Socially, yes.  However, the progress towards integration, on evaluating an individual by his or her merits and not by any other physical characteristic or socioeconomic background continues to rapidly advance.  President Obama's election represents and interesting milestone in the advancement of freedom and exercise of rights.  The fact that he could run and be elected because he was presumably the best person for the job is reflective of how far the US has come; prior candidates, such as Jessie Jackson, base a large part of their campaign on racial issues.  In other words, in this last contest for the Office of the President, skin color had a minor role.  Of course, there were those who voted for and against because of that, but it was mostly understood that the man's color had little to do with how well he could handle the job.

As the US becomes more and more a meritocracy, issues of color, culture, race, gender, sexual orientation, etc will become more and more moot.  This, in contrast to the long sweep of history, where whole cultures for centuries were legally subverted, one by another, and race, color and culture was the defining characteristic between master and slave.