The shahada is the Islamic creed; a single sincere recitation of it makes one a Muslim.
Indeed, a single public recitation of it, such as Senator Obama did with the New York Times, is in some ways very much like answering an altar call to publicly profess one's faith. However, instead of the often free-form version used in Christianity, in Islam it is a set formulation.
Currently, the Wikipedia article seems to be reasonably accurate: http://en.wikipedia.org/wiki/Shahada. However, it is subject to completely random edits, so while it may now be acceptable, unfortunately it may not stay so.
There is currently a reference in the Wikipedia article to http://www.al-islam.org/reflectionsnewmuslim/8.htm; this is an Islamic website, and it repeats the same information ("You must state this belief to God. This is called Shahada and is stated as follows: [ ... ] If you state those two things with belief and conviction, you are Muslim"); nor is this somehow unique --- the significance of reciting the shahada is very old (history records many instances of forced conversions by simply having a prisoner recite the shahada, such as was done during the Crusades.)
But in Senator Obama's case, it goes beyond anything like that. On his own initiative, as described by Nichola Kristoff in this way:
Mr. Obama recalled the opening lines of the Arabic call to prayer, reciting them with a first-rate accent. In a remark that seemed delightfully uncalculated (it’ll give Alabama voters heart attacks), Mr. Obama described the call to prayer as “one of the prettiest sounds on Earth at sunset.”
(From Nicholas Kristof's column Obama: Man of the World in the New York Times of March 6th, 2007.)
Now here's something interesting: this column is currently not available from the New York Times. I have to wonder if they have decided to try to scrub this damning interview?
Thank you.
That’s exactly what I thought.