From an inaugural press release ...

On January 20th, President-elect Barack Obama will take the oath of office using the same Bible upon which President Lincoln was sworn in at his first inauguration. The Bible is currently part of the collections of the Library of Congress. Though there is no constitutional requirement for the use of a Bible during the swearing-in, Presidents have traditionally used Bibles for the ceremony, choosing a volume with personal or historical significance. President-elect Obama will be the first President sworn in using the Lincoln Bible since its initial use in 1861.