I was a jelly donut
This topic has frequently been mentioned here – for instance in this 2005 entry. But it refuses to die the death. It is really entrenched in the USA.
Paper Cuts talked to Michael Jennings, the chairman of the German department at Princeton University.
After you wrote to me, I did a bit of informal research myself — talking to lots of friends in Berlin. And their responses were all over the map. Certainly the most common and accepted way to say “I’m a resident of Berlin” is “Ich bin Berliner,” i.e. without the indefinite article. But, for many speakers, it is by no means incorrect or ungrammatical to say “Ich bin ein Berliner.” Some of my respondents in fact applauded Kennedy on his nuanced use of German, since for them the sentence without the indefinite article implies that the speaker is a native Berliner, while the sentence with “ein” suggests either more recent residence in Berlin or even solidarity with its inhabitants (which was clearly Kennedy / Sorenson’s intention).