He should wait until January 20th at 11:59 AM Eastern Time. But no, he’s in a rush.

Obama resigning Senate seat as of Sunday
(Amanda Terkel, Think Progress, 13 Nov 2008)

Today, President-elect Barack Obama announced that he will be giving up his Senate seat, effective on Sunday. “It has been one of the highest honors and privileges of my life to have served the people of Illinois in the United States Senate,” Obama said in a statement. The Chicago Tribune notes that the news puts added pressure on Gov. Rod Blagojevich (D) to name a replacement for Obama, who is currently the only African-American senator.