The circumstances of McCain's birth unqusetionably make him an American citizen.
They are not sufficient to certainly make him a natural born citizen for the purposes of becoming president.
By virtue of being born on foreign soil he has the potential of divided loyalties or conflicting affinities.
Suppose what's happening in Honduras right now was happening in Panama? If McCain was the president, no action he could take or avoid would be regarded as neutral, would it?
Exactly the situation the Founding Fathers specifically tried to render impossible by requiring that that one office be held by someone who is first, foremost, and ONLY an American to the very core of his being.
You are incorrect, the Constitution makes no mention of birth on foreign soil disqualifying a person from the office of President. Regardless of the opinions of the founders, the Constitution is silent on the matter.
All US Presidents born prior to 1783 were dual citizens of both the US and Great Britain. Neither the Treaty of Paris, nor any Act of the Parliament of Great Britain revoked their British citizenship.
Dual citizenship does not effect eligibility for the Presidency. It is a political question, not a legal one. It is an issue for the voters to decide.
Legally when a dual national is in the United States, he is an American, period. His second nationality has no legal status under US law and is ignored.
Also, with travel being what it is, it is possible that a person may be born in the US to two US citizen parents and still aquire another nationality, even if that individual does not want it. This can happen if a country does not recognize US naturalization. Some countries do not allow their citizens to renounce citizenship. Naturalized Americans who wanted to dump their old citizenship could find that they and their children are stuck with an unwanted second citizenship.