Birth certificates issued in the era in which Obama was born did not generally contain a baby's SS #. Until relatively recently, an American citizen wasn't expected to apply for a Social Security number as a baby. The most common practice was that a Social Security number was applied for in preparation for employment.
I am about BO's age, and I can state that this is correct. I did not have my SS number printed on my birth certificate. As I recall, I applied for the card around the time I was in college.