PhoneArena stacks up a preview Samsung Galaxy S II unit to demonstrate how the Galaxy S sequel takes the "thinnest smartphone" crown from the iPhone 4. We also compare it with the 0.39" (9.9mm) thick Galaxy S and showcase how remarkably thin it is, even next to the 0.37" (9.3mm) iPhone 4. The Samsung Galaxy S II is 0.33" (8.49mm), which is less than even the Sony Ericsson Xperia arc, which is 0.34" (8.7mm) in its thinnest part…