The Samsung Galaxy S II is 8.49mm thick. In comparison, the iPhone 4 is 9.3mm thick. The iPad 2 is only 8.8mm thick.
Of course, there is that tiny, tiny fine print: "Thickness of the device may differ by country or carrier."
Watch a video on the Samsung Galaxy S II, showing off the thinness, below. There are a couple of older ones, as well.