Last time I looked into it, polygraph examinations were only about 85% reliable and that's with a highly trained examiner. The examinee has to meet some criteria as well such as never have been given an exam before.
Since we don't know if the polygraph was in fact given, much less if it was properly given by a professional examiner, we might suspect this to be a fraud.
I'm not a fan of Obama, but stupendous stories like this are most often lies.
On the other hand, since McCain has spoken at length about his serial infidelities, I have pretty strong confidence in their validity.