According to TexasDarlin, the evidence is that Soetoro is still his legal name from childhood (c. 1966), and his birth name Obama has become his alias.
http://texasdarlin.wordpress.com/2008/08/12/is-his-legal-name-obama-or-soetoro-act-now/
Texas Darlin says that Barack Hussien Obama is his legal name ISN'T IT.? They are going under the theory that Barack Hussien Obama was his birth name.
I am not sure that Obama was ever his legal last name. There is no marriage license to be found for Obama Senior and his mother Stanley Ann Dunham, and Barack Jr. in his own book says he never pressed the issue as to where, when or even if his mother was married.
Since a lot of us think the BC put our for Barack Hussien Obama is fake I believe he was an illegitamate child and his last name on the BC would have been his mothers last name, Dunham, (Thats the way it was back then) I think when she married Lolo Soetoro he then adopted Barack (Barry) and the original BC under Dunham was sealed. The reason I believe this is because if Barack Hussien Obama's name was on the birth certificate he would have had to give up ALL rights to the child in order for Soetoro to adopt him and there would be a record showing that he did that.