Not yet, it is currently being built.
You can try CyberPort in Friedrichstraße (U2, U6 - Stadtmitte) or Gravis (U2 - Ernst-Reuter-Platz) or many of the MediaMarkt or Saturn outlets (Alexanderplatz - U2, U5, U8 or S5 or S7 - has both) to get unlocked iPhones.
Maybe there is even an iPhone store on "Unter den Linden", which is right in the centre of Berlin and not far away from Friedrichstraße.
Also know, that the iPhone 5 is not yet available here as far as it seem.
And avoid the Simon-Dach-Straße, it is full of drunken tourists.