Yes it is. The office of President requires that you have loyalty to ONLY the United States. The founding fathers took that into account when requiring Natural Born Status. Which also includes not having citizenship in another Nation. Show me where, in the Constitution, there is a mention of dual citizenship being a bar to serving as President. There is no such restriction. Dual citizenship may be a political bar to the Presidency, in that people might be less likely to vote for a dual citizen for President, but it is not a Constitutional bar.
Like I wrote upthread, my son is a natural born citizen of the US, but he is also a citizen of at least 2 other countries. All perfectly legal, and he will be Consitutionally qualified to be President.
Yes, if those countries have reciprocity with the US, Dual citizenship is allowed. Indonesia does NOT. Obama would have had to renounce his citizenship in the US to become a citizen there. Its as simple as that. You can't go and renounce your citizenship and then get it back as if it never happened, thats why he became a Naturalized rather than natural Born Citizen. The founding Fathers saw it as an issue of Loyalty.