According to Indonesian law, Obama was too old to become an Indonesian citizen through adoption:
Law No. 62 of 1958, Law on the Citizenship of the Republic of Indonesia
Article 2.
(1)A foreign child of less than 5 years age who is adopted by a citizen of the Republic of Indonesia acquires the citizenship of the Republic of Indonesia, if such an adoption is declared legal by the Pengadilan Negeri at the residence of the person adopting the child.
http://www.unhcr.org/refworld/country,,,LEGISLATION,IDN,4562d8cf2,3ae6b4ec8,0.html
So, in order to travel on an Indonesian passport (should such have ever happened), he would have had to become an Indonesian Citizen, forsaking his US Citizenship in the process?