For what its worth:
"Until the 1970's the editors were picked on the basis of grades, and the president of the Law Review was the student with the highest academic rank. Among these were Elliot L. Richardson, the former Attorney General, and Irwin Griswold, a dean of the Harvard Law School and Solicitor General under Presidents Lyndon B. Johnson and Richard M. Nixon.
That system came under attack in the 1970's and was replaced by a program in which about half the editors are chosen for their grades and the other half are chosen by fellow students after a special writing competition. The new system, disputed when it began, was meant to help insure that minority students became editors of The Law Review."
First Black Elected to Head Harvard's Law Review - The New York Times
Also, the Law Review at Harvard has produced one US President and three US Supreme Court Justices.
Regardless, making it onto Law Review at any Law School, let alone Harvard, is a major accomplishment.
Whatever Obama did to get there, he was selected above his peers.
Here's an interesting article:
"For better or worse, voters have taken an interest in candidates' grades since 1999, when the New Yorker published President Bush's transcript at Yale and disclosed that he was a C student. Mr. Bush had never portrayed himself as a brain, but many were surprised to learn the next year that his opponent, Vice President Gore, did not do much better at Harvard despite his intellectual image. When Senator Kerry's transcript surfaced, reporters found that he actually had a slightly lower average at Yale than Mr. Bush did."
Obama's Years at Columbia Are a Mystery - September 2, 2008 - The New York Sun
This taken with most people are going to use anything positive to become President, leads me to believe the grades were average (still saying alot being Harvard and Columbia) and he probably got onto the Law Review because of the student vote.
Except that average grades wouldn't yield admission.