Two possibilities come to mind. (1)He may have not been born in the US at all, but in Kenya, which would make him constitutionally ineliegible to be POTUS. (2) He may have information on his birth certificate that would be embarrassing to disclose, like, for example, he was born out of wedlock or his father was someone other than Barack Obama Sr. In either case, such disclosures would be legally and/or politically disastrous to him.
Before people become consumed by the hysteria that we are about to elect an anti-american, non-citizen, communist, shiite sunni baathish muslim, closet-terrorist antichrist to the highest position in this great country, I highly recommend that everyone pause a moment, take a breath, and examine the facts:
http://www.factcheck.org/elections-2008/born_in_the_usa.html
A proud American and veteran.